There are a few explanations of the differences between WSS 3.0 and MOSS out there but this is by far the best I have come across.
http://sharepoint.microsoft.com/blogs/GetThePoint/Lists/Posts/Post.aspx?ID=4
Suzanne Ross who writes this SharePoint blog manages to cover the basics of SharePoint in a very strightforward and easy to understand way. I will definitely be pointing some of our users towards her blog posts in the future!