What is a Computer?Edit
If you are not reading this from printed paper, you will – without a shadow of a doubt – be using a computer (most likely taking the form of a desktop, laptop or portable mobile device) to read this WikiBook. Pretty much everything around you that's mechanical is a computer. You may say, "No way! My old wooden grandfather clock is not a computer," but you would be wrong — computers are 'general purpose machines that process data according to a set of (temporarily or permanently) internally stored instructions.'
A computer (or computing device) is, fundamentally, 'a programmable general purpose electronic device capable of performing multiple functions.' Or, in non-nerd terms, 'anything that performs defined functions automatically according to instructions it has already been given.'
So, that clock is indeed a computer: its 'set of instructions' is its internal timekeeping mechanism, cogs, springs, weights and all. The fact that its program happens to be one which is predictable in its expected output makes no difference (the gradual rotation of the hands around the clock face every twenty-four hours, or the fact that it self-regulates its output – the hands' movement – to once per second).
Whilst the example of a clock may be more reminiscent of the very first computer (Charles Babbage's Difference Engine) the concepts employed are identical. Practically anything electromechanical could be considered a 'computer'.
This Book's definition of a ComputerEdit
When most people talk about 'computers', they are implicitly referring to Personal Computers (PCs) - those wondrous devices that allow us to communicate, read, write and learn with the help of about a dozen common components all working harmoniously to send, receive, process and display information of every type imaginable. For consistency, whenever this Book mentions a 'computer', it is referring to a Personal Computer (to avoid any confusion possibly arising from the more specialist 'computing systems' or 'computational devices'.)
But what's in a computer? Read on to find out...
For those requiring rather more information - Computer Hardware