On a typical home network, where computers are all connected in peer-to-peer fashion they all have the same 'status', and operate independently of one another. Each one acts as a 'server' and a 'client', depending on what you're actually doing, eg pulling or pushing files from one machine to another.
In a typical large corporate network (a proper Server-Workstation setup), you will have a central server (maybe more than one) to handle user data files, user profiles and log-on scripts, maybe email etc etc.
So the word 'server' has different meanings depending on the type of network you're talking about. The good news is that, for a small home peer-to-peer network you don't really have to worry about it too much. Its just part of the learning curve (and I'm not at the top by any means). If you want to probe a little deeper into networks then OS X is probably the most painless OS to use. And finally, if you've got your iBook talking to your PC then pat yourself on the back - you're a network engineer!
