📖 Programmer's Bible
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What is a Computer?

⏱ 7 min read

In one sentence: A computer is a machine that follows a list of instructions, called a program, to take in information, work with it, keep it, and give back a result.

Why should I care?

You are using a computer right now. If you read this on a laptop, that is a computer. If you read it on a phone, that is a computer too. Many smaller things also have a computer inside them: microwave ovens, remote controls, cameras and thermostats (the controls that keep a room warm or cool).

A programmer writes instructions for computers. So the first step is to know what a computer is and what it can do.

Analogy

Think of a cook in a kitchen, following a recipe.

  • The cook receives ingredients: flour, eggs, water.
  • The cook follows the recipe, one step at a time: mix, heat, wait.
  • The cook keeps the food in use on the table, and the rest in the fridge for later.
  • At the end, the cook gives you a finished dish.

A computer is similar. It receives information, follows a list of instructions, keeps information, and gives back a result. The list of instructions is called a program, like the recipe.

Where the analogy breaks down: a cook understands the recipe. If a step is wrong ("add 100 eggs"), the cook notices and stops. A computer does not understand anything. It follows each instruction exactly, even a wrong one. Also, a recipe step like "make a sauce" is big. A computer's instructions are tiny, like "add two numbers", and a computer can follow billions of them every second.

How it actually works

Every computer does four kinds of jobs:

  1. Input: information goes in. You press a key, move a mouse, or touch a screen.
  2. Processing: the computer works with the information by following instructions. For example, it adds numbers or compares two words.
  3. Storing: the computer keeps information, so it can use it now or later.
  4. Output: a result comes out. Letters appear on the screen, or a printer prints a page.

In this picture, information flows from input, through processing, to output. Processing also sends information to memory and storage, and gets it back:

The four jobs of a computerInput, such as a keyboard, mouse or touch screen, goes to processing, which follows the program's instructions. Processing gives a result to output, such as a screen or printer. Processing is also connected both ways with memory and storage, which keep information and programs.

Input: keyboard, mouse, touch screen

Processing: follows the program

Output:
screen, printer

Memory and
storage

The part that does the processing is called the processor, or CPU (short for Central Processing Unit). One computer course describes it this way: it "acts like a brain: follows the instructions in the code". (Code means the instructions of a program.) You will learn more in CPU.

Each instruction is very small. The same course gives examples: "adding two numbers or testing if a number is equal to zero". A program such as the Firefox web browser is made of millions of these small instructions. When a computer follows a program's instructions, we say it runs the program. The course gives an example speed: 2 billion instructions per second.

How can adding numbers help with words? Inside a computer, each letter is stored as a number. So comparing two words is really comparing numbers. You will see how in Text as Numbers: ASCII to Unicode.

A computer stores information in two places, like the cook's table and fridge:

  • Memory (also called RAM, short for Random Access Memory) holds the program that is running and the information it is using right now. It is temporary: when the power is switched off, it is lost. You will learn more in Memory (RAM).
  • Storage keeps information for a long time, as files and folders. It keeps them even when the power is switched off. See Storage: HDD and SSD.

Two more words you will see often:

  • Hardware is the physical parts of a computer: the parts you can touch.
  • Software is the programs that run on the computer. See Software and Programs.

The same course compares them to a piano (the hardware) and music (the software). One piano can play many songs. In the same way, one computer can run many different programs. For example, the same phone can run a camera program, a map program and a calculator program.

How it came to be

For centuries, the word "computer" did not mean a machine. It meant a person. The Computer History Museum describes them as "low-paid clerks doing simple math" (clerks are office workers). According to the Oxford English Dictionary, the oldest known use of the word is in a book from 1613.

In the 1940s, people began to build electronic computers: machines made of electronic parts, which calculate much faster than a person. In the United States, a team of human computers set up and programmed one of these machines, called ENIAC. According to Penn Engineering, one ballistics calculation (working out the path of a bomb or missile) took 12 hours on a hand calculator. ENIAC could do it in 30 seconds.

In the following years, electronic computers took over the work of human computers. The word "computer" kept its old meaning, a person, until the middle of the 1900s. To read about the tools people used before electronic computers, see Before Computers: the Abacus and Mechanical Calculators.

🧪 Try it in your browser

You will watch your computer do all four jobs in a second or less.

  1. Stay on this page.
  2. On Windows and most other computers, press Ctrl + F. On a Mac, press Command (the ⌘ key) + F. This works in Chrome, Edge and Firefox. A small search box opens. On a phone in Chrome, tap the menu with three dots, then Find in page.
  3. Type the word computer into the box.
  4. Press Enter. (On a phone, tap the search button.)

What you should see: your browser jumps to a place where the word "computer" appears on this page, and highlights it with a colour. Depending on your browser, it may also highlight the other places, and show a number: how many times it found the word.

What happened:

  • Input: you typed a word.
  • Storing: the text of this page was already kept in your computer's memory.
  • Processing: following the browser program's instructions, your computer looked through all the text on the page for your word.
  • Output: the highlighted words appeared on your screen.

Common misunderstandings

  • "A computer is a box on a desk." No. Laptops, phones and tablets are computers too. Small computers also run inside microwave ovens, remote controls, cameras and thermostats.
  • "Computers think for themselves." No. A computer follows instructions that people wrote. One computer course says a computer "requires a human to tell it what to do". It is fast, not wise.
  • "A computer can only do one kind of job." No. The same computer can run many different programs, like one piano playing many songs. A computer can even run several programs at the same time.

Recap

  • A computer is a machine that follows a program: a list of very small instructions.
  • Every computer does four jobs: input, processing, storing and output.
  • The word "computer" first meant a person who did calculations. Electronic computers took over that work in the middle of the 1900s.

Sources

Written with AI · Reviewed by AI · Not yet checked by a human

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