What is a computer?

It's a little less obvious than it seems, actually. A computer is any machine that deals with information. That's a pretty broad definition, and it includes things like calculators, TVs, digital music players, mobile phones and even cars.

Each of these is specialised—it's only designed to do one specific thing: manipulate numbers; show moving pictures with sound; play music; talk to other people; and... deal with car stuff..., respectively. (What does an in-car computer do, anyway?)

Usually, though, when someone talks about a “computer” they mean a personal computer.

So what’s a personal computer, then?

Unlike the computers inside calculators and videos and things, a PC is a generic computer—it's designed to be able to do lots of different things: you can draw pictures with a PC, read prose, play games, record sound, play video, calculate pi, cure cancer...

A computer is the whole system: it's made up of lots of different parts, each with its own function. The central processing unit (CPU) is the main brain—it does most of the thinking; the memory (or “RAM”) stores the things the CPU is thinking about at the moment; there's also usually a long-term storage device, typically a hard disc, which holds everything that needs to be kept (so, all your stuff); there'll be a means for you to talk to the computer, typically a keyboard and mouse; and a means for the computer to talk to you, typically a screen (or “monitor”) and some speakers. (I'll go into more detail about each of these later.)

Convergence

Recently (in the 2000s), things that weren't generic computers have started to become more generic—everything's converging towards being a computer. For example, mobile phones used to only be able to make phone calls and send text messages; now they can also store notes, play music and video, browse the web, and transfer all kinds of information to many other devices.

The computing power of small portable devices is fast approaching the level that a full-blown desktop PC might have had not too long ago. My computer's about ten years old; apart from their smaller screens, many modern “smartphones” have very similar technical specifications to it.

It's this abundance of computing power that's allowed specialised devices to become more generic and more capable; and so a lot more things can nowadays be usefully considered a “computer”.