What Computers Actually Do | introduction to computer science Guide

Most people use computers every day but rarely stop to ask a simple question:

What is a computer actually doing?

When you send a message, open a game, watch a video, or run a program, a lot is happening behind the scenes. But surprisingly, almost everything a computer does can be explained with a very simple model.

Understanding this model will make everything else in computer science easier to understand.

What a Computer Really Is

At its core, a computer is just a machine that processes information.

That’s it.

It receives information, performs operations on that information, stores some of it, and produces results.

This applies to nearly every digital device you can think of.

Device	What it Processes
Laptop	Documents, programs, web pages
Smartphone	Messages, apps, photos
ATM	Banking transactions
Game console	Game logic, graphics
Smart thermostat	Temperature data

Even massive systems like search engines or social networks are just very large computers processing huge amounts of information.

The Four Things Every Computer Does

No matter how simple or complex the system is, computers always perform four fundamental tasks.

graph LR
  A[Input] --> B[Processing]
  B --> C[Storage]
  B --> D[Output]

Let’s break these down.

Input — Getting Information Into the System

Before a computer can do anything, it needs input.

Input is simply data entering the system.

Examples of input include:

Input Source	Example
Keyboard	Typing a message
Mouse	Clicking a button
Camera	Taking a photo
Microphone	Recording voice
Network	Receiving data from the internet

For example, when you search for something online:

You type a query.
Your browser sends that text to a server.

That text is input to the system.

Processing — Doing the Work

Once the computer receives input, it needs to do something with it.

This is called processing.

Processing is performed by the CPU (Central Processing Unit), which executes instructions from programs.

Examples of processing include:

Adding numbers
Sorting data
Rendering images
Running game physics
Deciding which video to recommend next

Programs are essentially lists of instructions that tell the computer how to process information.

graph TD
  A[Program Instructions] --> B[CPU Executes]
  B --> C[Results Produced]

You’ll learn much more about programs and instructions later in the Programming guide.

Storage — Remembering Information

Computers also need to store data.

Without storage, a computer would forget everything the moment it finished processing.

There are two main kinds of storage:

Type	Example	Purpose
RAM	Memory used while programs run	Temporary storage
Disk	SSD or HDD	Long-term storage

For example, when you open a game:

The game files are stored on your disk.
They are loaded into RAM.
The CPU processes them while the game runs.

Storage allows computers to remember things between operations.

Output — Showing the Result

Finally, the computer produces output.

Output is how the computer communicates results back to the user or another system.

Examples include:

Output	Example
Display	Text, images, videos
Speakers	Music or sound effects
Printer	Printed documents
Network	Data sent to another computer

If you type a message and press send:

The keyboard provides input
The program processes the message
The message is stored
The message appears on the screen

That last step is output.

A Real Example: Watching a Video

Let’s see how these four steps work in a real situation.

Imagine you open a video on a streaming platform.

graph TD
  A[Click Video] --> B[Request Sent to Server]
  B --> C[Server Processes Request]
  C --> D[Video Data Retrieved]
  D --> E[Video Sent to Your Device]
  E --> F[Video Displayed on Screen]

Behind the scenes:

Step	What Happens
Input	You click the video
Processing	Servers locate the video
Storage	Video file retrieved from storage
Output	Video appears on your screen

Even extremely large systems still follow the same simple pattern.

Programs: The Instructions Behind Everything

So who tells the computer what to do?

Programs do.

A program is simply a set of instructions written by a programmer.

For example, a very simple calculator program might do something like this:

read number A
read number B
add A and B
display result

These instructions are written in programming languages like:

C
Python
JavaScript
Java

Programming languages exist because writing raw machine instructions would be extremely difficult for humans.

You’ll explore this in depth in the Programming Guide.

Why This Model Matters

You might be wondering why this simple model is important.

It turns out that nearly every concept in computer science builds on this idea.

When you later study:

Operating Systems → they manage how programs use CPU, memory, and storage
Databases → they organize stored information efficiently
Networking → they move input and output across machines
Programming → it defines how processing should happen

Understanding how computers input, process, store, and output information gives you a mental framework for understanding all of these systems.

What Comes Next

Now that you understand what computers fundamentally do, the next step is understanding what computers are made of.

In the next chapter we will explore the difference between hardware and software, and how they work together to run modern computer systems.