Short answer
- Inside a computer case you will find a small set of parts that work together as one machine. The motherboard is the main circuit board that connects everything. The CPU, or central processing unit, carries out the instructions from your programs. RAM is short term memory that holds what the computer is working on right now. Storage, a hard disk drive or a solid state drive, keeps your files and programs even when the power is off. The graphics hardware turns data into the picture on your screen, the power supply converts wall electricity into the form the parts can use, and fans and heat sinks keep everything cool enough to run. Each part has one main job, and no part can do the work of the whole machine alone. The CPU needs RAM to hold its working data, RAM needs storage to load programs from, and all of them need the motherboard and the power supply to reach each other and to receive electricity. That teamwork is what this article explains, part by part, in plain language. If you want the wider definition of what a computer is first, see [What Is a Computer](/technology/what-is-a-computer/).
Before You Look Inside
Opening a desktop computer is simple in principle: unplug the power cord, remove a side panel, and the parts are in front of you. In practice, a few precautions matter. Always unplug the computer first, because parts inside can still carry power even when the machine is switched off. Touch a bare metal part of the case before touching anything else, so static charge on your body has somewhere harmless to go. Static electricity can damage sensitive chips, and a small spark you barely feel can be enough.
Do not open a computer that is still under warranty unless you have checked the warranty terms, because opening the case can void coverage on some machines. Laptops are a different matter: their parts are packed tightly, many are soldered in place, and opening one usually needs special tools. This article describes a typical desktop computer, because that is the machine where the parts are easiest to see and to tell apart. The same parts exist in a laptop, only smaller and harder to reach.
You do not need to open your own computer to understand this article. The tour below works from photographs and descriptions, and every part is explained by what it does, not by how to remove it.
The Case: The Box That Holds It All
The case is the metal or plastic box that holds every internal part. It does three jobs. It protects the parts from knocks, dust, and accidental contact. It holds them in fixed positions, so connectors stay seated and cables stay routed. And it shapes the airflow, with vents and fan mounts that guide cool air in and warm air out.
On the back of a desktop case you will see the ports: USB sockets, audio jacks, a network socket, and the video outputs from the graphics hardware. Those ports are not separate parts. They are the outside ends of connectors that sit on the motherboard or on an expansion card inside. The power socket on the back belongs to the power supply unit, which is also mounted inside the case.
Cases come in many sizes, from large towers with room for several drives and cards to small boxes that hold a compact motherboard and a single drive. The size of the case limits what can fit inside it, which is why a large tower can hold more storage drives and a bigger graphics card than a small desktop. The case itself does no computing. It is the frame that makes the computing parts into one machine you can carry, plug in, and use.
The Motherboard: The Board That Connects Everything
The motherboard is the largest circuit board inside the computer, and it is the part everything else plugs into. It is a flat board covered with copper traces, which are thin printed pathways that carry electrical signals between components. TechTarget describes it as the main printed circuit board in a computer and the central communications backbone through which all components connect. That description is a good one: the motherboard is less a part with a single job and more the meeting place where every other part does its job.
Look at a motherboard and you will see a set of sockets and slots. The CPU sits in a socket, usually near the center, under a cooler. Long slots beside it hold the RAM sticks. Other slots accept expansion cards, including the graphics card. Connectors along the edges take cables from the storage drives, the power supply, the case buttons, and the case fans. The ports on the back of the computer are soldered to the motherboard or to cards fitted into it.
The motherboard also carries a chipset, a set of chips that manages traffic between the CPU, memory, storage, and the slower connections such as USB and audio. Modern boards also carry firmware, low level software stored in a chip on the board, that starts the computer and hands control to the operating system when you switch the machine on. That firmware used to be called the BIOS, and newer boards use a system called UEFI. Either way, its job is the same: wake the hardware up in the right order and find the drive that holds the operating system.
Because the motherboard decides which CPU fits, how much RAM can be installed, and which storage and graphics connections are available, it sets the limits of the whole machine. Two computers can have the same processor and still behave differently if their motherboards offer different memory speeds or different storage connections. When people upgrade a computer, the motherboard is often the part that decides whether a new component will fit at all.
The CPU: The Processor That Does the Thinking
The CPU, or central processing unit, is the chip that carries out the instructions of your programs. Intel defines it as the electronic circuitry that executes the instructions comprising a computer program, and Computer Hope puts it simply: the CPU handles all instructions it receives from hardware and software running on the computer. When you open a document, click a link, or save a file, the CPU is the part that works through the steps those actions require.
Inside the CPU are two main kinds of circuitry. The arithmetic logic unit, or ALU, performs calculations and logical comparisons, such as adding numbers or deciding whether one value is larger than another. The control unit directs the flow of work, fetching each instruction from memory, working out what it means, and sending the right signals to the right parts of the chip. This repeating cycle of fetch, decode, and execute is the basic rhythm of all computing, and it runs billions of times every second in a modern processor.
Speed is measured in gigahertz, written GHz. One gigahertz is one billion cycles per second, so a processor rated at 3 GHz can run three billion cycles each second. Clock speed is not the whole story, though. Modern CPUs also contain several cores, which are smaller processing units inside the same chip that can work on different tasks at the same time. A four core processor can handle four threads of work in parallel, which matters more for everyday use than a small difference in clock speed. Cache memory, a small amount of very fast memory built into the CPU itself, holds the data the processor is most likely to need next, so it does not have to wait for RAM.
The first commercially available microprocessor, the Intel 4004, was made in 1971 and contained about 2,300 transistors. Modern processors contain billions of transistors, which are tiny switches that turn on and off to represent data. That growth in transistor count, along with faster clocks and more cores, is the main reason a modern computer can do in a second what an early machine needed minutes or hours to finish. If you want the history behind that change, including why no single person can be called the inventor of the computer, read Who Invented the Computer?.
The CPU sits under a cooler, so you will rarely see the chip itself when you open a case. What you see is usually a metal heat sink with a fan on top, or a flat plate connected to a liquid cooling loop. The cooler is there because the CPU produces a great deal of heat while it works, and the cooling section below explains why that matters.
RAM: Short Term Memory
RAM stands for random access memory, and it is the computer’s short term memory. When you open a program, the computer copies the parts of that program it needs from storage into RAM, because RAM is far faster to read and write than any storage drive. The CPU then works with the data in RAM. Crucial describes RAM as the temporary memory that gives applications a place to store and access data on a short term basis, and notes that it is volatile, which means its contents are lost when the power is turned off. That is the key difference between RAM and storage: RAM forgets everything when the power goes, and storage remembers.
RAM is called random access because the CPU can reach any part of it in about the same amount of time, without reading through everything that came before. That is very different from an old cassette tape, where reaching a song near the end meant winding through the whole tape. In a computer, that direct access is what makes switching between programs feel instant: the CPU can jump straight to the data it needs, wherever it sits in memory.
RAM comes as slim sticks, called modules, that plug into slots on the motherboard. The memory in most desktop computers is a type called DRAM, or dynamic RAM, which stores each bit of data as a tiny electrical charge that must be refreshed regularly to stay readable. A faster, more expensive type called SRAM, or static RAM, is used in small amounts inside the CPU as cache memory. You do not need to remember those names to use a computer, but they explain why RAM is sold in different types and speeds, and why a stick that fits one motherboard may not fit another.
Capacity is measured in gigabytes, written GB. Eight gigabytes is enough for ordinary browsing, email, and documents on many machines, while heavier work such as video editing or running many programs at once benefits from more. Adding RAM is one of the simplest upgrades for a slow computer, because more RAM means the computer can keep more programs ready at once instead of shuffling data back to the slower storage drive. For the full comparison between RAM and the permanent memory that survives a power cut, see Difference Between RAM and ROM.
Storage: Where Your Files Live
Storage is where the computer keeps everything it needs to remember: the operating system, your programs, your documents, your photos, and your settings. Unlike RAM, storage is non volatile, which means it keeps its contents when the power is off. That is why your files are still there when you switch the computer back on.
There are two main kinds of storage in modern computers. A hard disk drive, or HDD, stores data on spinning metal discs called platters, coated with a magnetic layer. A small arm with a read/write head floats just above the surface and magnetizes tiny areas to record ones and zeros. Intel describes the head as working much like the needle of a record player, moving across the spinning disc to reach the data it needs. Because the discs must spin up and the head must move into position, a hard drive is slower to reach data than the alternatives, but it offers a large amount of space for a low price per gigabyte.
A solid state drive, or SSD, stores data in flash memory chips and has no moving parts at all. Intel notes that SSDs use flash based memory to store data, which gives them faster read and write speeds and lower access times than hard drives, at a higher cost per gigabyte. Because there is nothing to spin up and no head to move, an SSD can find any file almost immediately, which is why computers with SSDs start up faster and open programs more quickly. SSDs are also silent and better able to survive a knock, since there are no moving parts to damage.
Most new computers now use SSDs as their main drive, while hard drives still appear where large amounts of cheap space matter, such as in machines that store large video collections. Some computers have both: a smaller SSD for the operating system and programs, and a larger hard drive for files that are rarely opened.
Storage connects to the motherboard in a few ways. Older drives and many hard drives use a SATA connection, a cable that links the drive to the motherboard. Newer, faster SSDs often use NVMe, a protocol that lets the drive connect directly to the motherboard’s PCIe lanes, which are high speed data paths also used by graphics cards. Intel explains that NVMe provides higher throughput and lower latency than SATA, which is why NVMe drives are the fastest storage most people can buy. The shape of the drive matters too: many NVMe SSDs are small sticks that plug straight into a slot on the motherboard, with no cable at all.
Capacity is measured in gigabytes or terabytes, written TB, where one terabyte is about one thousand gigabytes. A typical new laptop or desktop might have 256 GB or 512 GB of SSD storage, while a hard drive in a desktop might hold one or two terabytes. Your storage fills up with programs, files, and updates over time, and a nearly full drive can make a computer feel slow, because the operating system needs free space to work comfortably.
Graphics: Turning Data into Pictures
Everything you see on your screen has to be drawn, pixel by pixel, many times a second, and that drawing is the job of the graphics hardware. The graphics processing unit, or GPU, is a specialized processor built for this work. NVIDIA describes the GPU as a processor that excels at executing thousands of threads in parallel, and explains that while a CPU is designed to run a small number of complex sequences as fast as possible, a GPU is designed to run thousands of simpler operations at the same time. That difference is why the two chips exist side by side: the CPU is good at making decisions one after another, and the GPU is good at doing the same calculation across many pixels at once.
Every computer has some form of graphics hardware. Many processors include integrated graphics, a basic GPU built into the same chip as the CPU, which is enough for browsing, documents, video playback, and ordinary office work. A separate graphics card is a larger board that plugs into a slot on the motherboard and carries its own GPU and its own memory. Dedicated cards are used for gaming, video editing, 3D design, and other work that draws complex images quickly, because they can handle far more pixels per second than integrated graphics.
The graphics card has its own memory, called video memory or VRAM, which holds the textures, frames, and image data the GPU is working on. A card also has its own cooling, usually one or more fans, because drawing millions of pixels many times a second produces a lot of heat. The monitor plugs into the graphics card, or into the motherboard when integrated graphics are in use, and the card sends the finished picture to the screen many times a second. A display that refreshes 60 times per second needs a new picture 60 times every second, and each picture may contain more than two million pixels, which gives a sense of how much work the GPU does without you noticing.
If you never play games or edit video, you may never need a separate graphics card. Integrated graphics have improved steadily, and for everyday use they are often all a computer needs. The graphics card is one of the easiest parts to upgrade, though, which is why it is often the first part people replace when a computer starts to struggle with newer games or heavier visual work.
The Power Supply: Electricity, Translated
The power supply unit, or PSU, is the metal box at the back of the case where the power cord plugs in. Its job is to convert the electricity from your wall outlet into the form the computer’s parts can use. Wall outlets supply alternating current, or AC, at a high voltage, while computer electronics run on direct current, or DC, at low voltages. Wikipedia describes the PSU as converting mains AC to low voltage regulated DC power for the internal components of a desktop computer, and Corsair explains that the PSU converts the high voltage AC from the outlet to the lower voltage DC the components require, then distributes it to the motherboard, CPU, GPU, storage drives, and everything else.
A modern PSU supplies several different voltages, called rails, because different parts need different amounts. The main rails are 12 volts, 5 volts, and 3.3 volts. The 12 volt rail does most of the heavy work, feeding the CPU and the graphics card through the motherboard, while the 5 volt and 3.3 volt rails serve drives, USB ports, and motherboard logic. Cables run from the PSU to the motherboard, to the graphics card, to the storage drives, and to the case fans, so every part gets the power it needs.
No power supply is perfectly efficient. Some of the energy it draws from the wall becomes heat instead of useful power, so a PSU must draw more from the outlet than it delivers to the parts. The 80 PLUS certification program measures this: a certified PSU converts at least 80 percent of the AC power it draws into usable DC power. A higher wattage rating means the PSU can deliver more power in total, which matters if you add a powerful graphics card or extra drives. A PSU that is too weak for the parts it feeds can cause crashes or prevent the computer from starting at all.
The PSU also contains safety features that shut the computer down if something goes wrong, such as a short circuit or a sudden spike in voltage. Inside the PSU are transformers, capacitors, and rectifiers that do the conversion work, but the PSU is one part you should never open yourself, because its capacitors can hold a dangerous charge even after the computer is unplugged. Unlike the other parts in this article, the power supply is sealed, and it should stay that way.
Cooling: Why Computers Need Fans
Every part described so far produces heat while it works, and heat is the enemy of reliable computing. Electrical resistance turns some of the energy flowing through a chip into heat, and the faster a chip works, the more heat it makes. If a chip gets too hot, it slows itself down to protect itself, and if it gets hotter still, the computer may shut down or the chip may be damaged. Cooling exists to carry that heat away before it causes trouble.
The simplest cooling device is the heat sink, a block of metal with many thin fins that sits on top of the CPU or another hot chip. The fins give the heat somewhere to spread out, so it can pass into the surrounding air. A fan mounted on the heat sink blows air across the fins, carrying the warm air away and bringing cooler air in. Together, the heat sink and the fan keep the chip within the temperature range it was designed for.
Most desktop computers have several fans: one on the CPU, one or more on the graphics card, and one or two in the case itself that move air through the whole box. Laptops use smaller fans and thinner heat pipes, which are sealed tubes that carry heat away from the processor to a radiator near the edge of the machine. Some high performance computers use liquid cooling, where a pump moves liquid through a block on the CPU and out to a radiator, but the idea is the same: move heat from the chip to somewhere it can escape.
Dust is the quiet enemy of cooling. Dust settles on fan blades and clogs the gaps between heat sink fins, which slows the air and traps heat. A computer that once ran quietly may grow louder over time as its fans spin faster to compensate, and a dusty machine may run hotter than a clean one. Blowing dust out with compressed air, with the computer unplugged, is one of the simplest maintenance jobs there is, and it is worth doing before assuming a slow or noisy computer needs a new part.
How the Parts Work Together
The parts above only make sense as a team, so it helps to follow one ordinary action through the machine. Suppose you double click a document to open it. The mouse sends a signal into the computer, and the CPU works out from that signal which program and which file you mean. You can read a full explanation of the mouse itself in What Is a Computer Mouse?, and a wider explanation of inputs and outputs in What Are Computer Inputs and Outputs?.
The operating system asks the storage drive for the document, and the drive reads the file from its platters or its flash memory. The file is copied into RAM, where the CPU can reach it quickly. The CPU runs the program’s instructions, fetching each one from RAM, decoding it, and executing it, billions of times a second. As it works, the program’s window is drawn by the graphics hardware and sent to the screen, while the power supply feeds every part with the electricity it needs and the fans keep the temperature under control.
Underneath all of this, everything is stored and moved as binary digits, or bits. A bit is a single 0 or 1, and eight bits together make a byte. NIST defines a byte as a sequence of eight bits, and that simple fact explains the units you see on every computer: a document measured in kilobytes, a photo in megabytes, a drive in gigabytes. The CPU, the RAM, and the storage all handle the same ones and zeros, just in different places and at different speeds.
When you save the document, the flow runs in reverse. The CPU sends the changed data from RAM back to the storage drive, where it is written to the platters or the flash memory so it survives when the power goes off. When you close the program, its space in RAM is freed for something else. Every click, keystroke, and file you open follows this same pattern: storage to RAM, RAM to CPU, CPU to screen, and back again.
What Is Not Inside the Case
The parts inside the case do the computing, but they are not the whole computer as most people use it. The keyboard, the mouse, the monitor, and the speakers sit outside the case, and they are called peripherals. They connect to the ports on the back of the case, and they are how you and the computer talk to each other. The mouse and keyboard send instructions in, and the monitor and speakers show or play results out. Without them, the parts inside the case would still run, but you would have no way to use them.
Some computers blur this line. A laptop packs the keyboard, the screen, and the trackpad into the same body as the internal parts, and an all in one desktop builds the screen and the computer into a single unit. Even then, the same division holds: the computing parts are inside, and the parts you touch and look at are the interface between you and them. A printer, a webcam, and an external drive are peripherals too, and they all connect through the same ports on the back of the case.
When a Computer Feels Slow
A computer that feels slow is rarely broken. More often, one part has become the bottleneck that holds the others back. If programs take a long time to open and the computer struggles with several things at once, the cause is often too little RAM or a hard drive doing the job a solid state drive could do faster. If the computer is loud and sluggish at the same time, dust in the fans or a cooler clogged with dust may be forcing the processor to slow down to protect itself. If the computer feels slow only when you open a game or edit video, the graphics hardware may be the part that needs help.
Working out which part is the bottleneck saves money, because it tells you whether an upgrade will help at all. Adding RAM will not fix a computer that is slow because its storage drive is failing, and a faster graphics card will not help a machine that is slow because it has too little memory. The steps in How to Speed Up a Slow Mac follow the same idea of diagnosing before fixing, and the order it suggests, checking the simple causes first, applies to Windows computers as well as Macs.
Frequently Asked Questions
What is the most important part inside a computer?
No single part is the most important, because the parts depend on each other. The CPU does the thinking, but it cannot work without RAM to hold its data, storage to load programs from, a motherboard to connect it all, and a power supply to feed it electricity. A weak link anywhere in that chain will slow the whole machine down.
What is the difference between RAM and storage?
RAM is short term memory that the CPU uses while it works, and it forgets everything when the power is off. Storage is long term memory that keeps your files and programs even when the power is off. RAM is much faster than storage, but storage holds far more, and storage is the only one of the two that remembers.
Do I need a graphics card?
It depends on what you do. Every computer has graphics hardware, but many computers use integrated graphics built into the CPU, which is enough for browsing, documents, and video. A separate graphics card is worth having for gaming, video editing, 3D design, and other work that draws complex images quickly.
Can I open my own computer to look inside?
On most desktop computers, yes, after unplugging the power cord and touching a bare metal part of the case to discharge static. Check the warranty first, and never open the power supply unit. Laptops are much harder to open safely and often need special tools, so they are best left to a repair shop unless you know what you are doing.
Why does my computer get hot?
Because every chip turns some of its electricity into heat while it works. Fans and heat sinks carry that heat away, and dust can make their job harder. A computer that gets hot will slow itself down to protect its chips, which is why cleaning dust out of the fans can sometimes make a slow computer feel faster.
What does the motherboard do?
The motherboard is the main circuit board that connects everything inside the computer. The CPU, RAM, storage, graphics card, and power supply all plug into it or connect through it, and its chipset manages the traffic between them. It also decides which parts fit, so it sets the upgrade limits of the whole machine.
Sources & references
- Computer Hope, "What Is CPU (Central Processing Unit)?": computerhope.com. CPU definition and components, the ALU and the control unit.
- Crucial, "What is RAM and what does RAM do?": crucial.com. RAM as temporary memory, volatility, DRAM and SRAM types.
- Intel, "HDD vs SSD for Gaming: How to Choose the Right Storage": intel.com. SSD flash memory with no moving parts, HDD spinning magnetic media, NVMe and SATA.
- National Institute of Standards and Technology, Computer Security Resource Center glossary, "Byte": csrc.nist.gov. A byte as a sequence of eight bits.
- NVIDIA, CUDA Programming Guide, "The Graphics Processing Unit": docs.nvidia.com. GPU parallel processing, thousands of threads, CPU and GPU design differences.
- TechTarget, "What is a motherboard?": techtarget.com. Motherboard as the main printed circuit board and central communications backbone.
- Wikipedia, "Central processing unit": en.wikipedia.org. CPU fetch decode execute cycle, first commercial microprocessor the Intel 4004 in 1971.
- Wikipedia, "Motherboard": en.wikipedia.org. Motherboard definition, chipset, firmware, and connectors.
- Wikipedia, "Power supply unit (computer)": en.wikipedia.org. PSU converts mains AC to low voltage regulated DC, ATX standard.
- Wikipedia, "Random-access memory": en.wikipedia.org. RAM volatility, DRAM and SRAM, random access.
