"Space... is big. Really big. You won't believe how vastly, hugely, mindboggling big it is. I mean, you may think it's a long way down the road to the chemist, but thats just peanuts to space."Douglas Adams, Hitchhiker's Guide to the Galaxy
A network is a group of computers that have been connected together. The Internet is a collection of networks. A collection of networks doesn't sound very interesting and, by itself, it isn't. The magic comes from what you can do once the networks are connected.
Before you can do anything with this collection of networks, though, they must all be doing things the same way. Imagine trying to pass a document from a network of IBM computers to a computer on a network of Macintoshes: almost everything about the two networks is different. They are using different microchips, different file structures and different software.
To cope with this, people have developed a set of protocols which set out the absolute minimum requirements for networks to be able to do things together. Any network that wants to work with another network must abide by these protocols. They form the basic glue that sticks the Internet together, so another way to define the Internet is a collection of protocols.
A third definition of the Internet is a collection of people working together. People have found that they want to go beyond the cooperation demanded by the protocols. They want to talk and work and play together, fall in love or hate one another, share and trade and build together. In short, they want to form communities.
And they want to do it fast! In just 30 years, the Internet has grown from four networks (Internet hosts) to over fifty-six million networks. That figure is currently growing exponentially, and there is no sign of it slowing down.
All these networks are used by people. I can't tell you how many people - no one knows. At the moment, no-one even knows how to make a good guess.
In 1969, the first four computer networks on the Internet were a Xerox, an SDS940 (whatever that is ), an IBM and a Digital (DEC). One of the main things about the Internet is that it allows all sorts of computers to talk to one another. Generally, it doesn't matter if you are using an IBM, an Apple or an Atari. There are super-computers, printers, coke machines, video cameras and radio broadcasters connected to the Net. In the near future, watch out for Pay TV, ATMs and your fridge to get connected.
I don't know if Xerox make computers any more, but I know that their photocopiers are connected.