This slide neatly sums up the main differences between Web 1.0, Web 2.0 and Web 3.0.
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.
If that sounds confusing, check out some of these excellent presentations that help you understand Web 3.0 in simple English. Each takes a different approach to explain Web 3.0 and the last presentation uses an example of a “postage stamp” to explain the “semantic web”.