Web 2.0 is so called second generation web based services like social networks, wikis, communication tools. For them online team-work and exchange between users are most important.
Despite, the term gives a hint for new version of the WWW, there are changes in the ways web standards are used, not changes in web standards themselves.
Some consider that Web 2.0 is basic marketing concept, which take online marketing communications in other dimension, with horizontally and vertically communication together. Most typically websites with Web 2.0 are Youtube, MySpace, Facebook.