Digital Inspiration explains the difference between web iterations in a clear and concise manner. There are six video presentations that lay it all out in simple and easily understood language. From the site:
Web 1.0 - That Geocities & Hotmail era was all about read-only content and static HTML websites. People preferred navigating the web through link directories of Yahoo! and dmoz.
Web 2.0 - This is about user-generated content and the read-write web. People are consuming as well as contributing information through blogs or sites like Flickr, YouTube, Digg, etc. The line dividing a consumer and content publisher is increasingly getting blurred in the Web 2.0 era.
Web 3.0 - This will be about semantic web (or the meaning of data), personalization (e.g. iGoogle), intelligent search and behavioral advertising among other things.
Found at iLibrarian