Good article from Tim O'Reilly on the definition of Web 1.0 and Web 2.0. Web 1.0 meaning the types of web sites that were part of dotcom fallout and Web 2.0 meaning the types of web sites that provide interaction; many community sites and community elements are a large contributor to the web 2.0 era and are provided in this article as examples.

http://www.oreillynet.com/pub/a/oreilly/tim/news/2005/09/30/what-is-web-20.html?page=1