For a complete understandng of the basic concepts of networking you could do a lot worse than having a look at this site
Networking Articles & Tutorials: Windows Vista / 2008 / 2003 / XP
The link is to the articles and tutorials section which should have information at a suitable level for all, with more advanced information available as you progress.
As Ken has said have a read and if you have any questions fell free to ask them here - it's why we're here
For a basic introduction to setting up a mixed OS network have a look at this page ...