A computer is a machine that follows instructions to work with information. It accepts input, processes it, stores it and produces output. A desktop PC, laptop and phone look different, but each follows this basic pattern.
Input, processing and output
Input can come from a keyboard, mouse, camera, network connection or a file already stored on the device. A program provides instructions for what to do with that input. The processor carries out those instructions, and the result might appear on a screen, play through speakers or be saved as a file.
For example, when you open a photograph, the computer reads the image file from storage, moves data into memory, processes it and sends pixels to the display. This happens quickly enough that the steps usually feel like one action.
The main parts
The central processing unit (CPU) executes instructions. Random access memory (RAM) holds information that programs need while they run. Storage, often a solid-state drive (SSD), keeps files and programs when the power is off. The motherboard connects these parts, while the power supply provides electricity. Graphics hardware prepares images for the screen.
These parts have different jobs. Adding storage gives you more room for files; it does not directly give running programs more working memory. Our guides to RAM and SSDs explain the difference.
Hardware and software
Hardware is the physical equipment. Software is the set of instructions it runs. The operating system, such as Windows, Linux or macOS, manages hardware resources and gives programs a common way to use them. A web browser is an application running on top of the operating system.
Neither side is useful in isolation. A program needs hardware to execute; hardware needs instructions to do something meaningful.
What happens at startup?
When you turn on a computer, firmware starts first. It checks and initialises hardware, then starts an operating system from storage. The operating system loads the services and interface you use to open programs. If the machine does not respond at all, see Computer won’t turn on.
The next useful concept is the CPU: the component that works through program instructions.