From Wikipedia: Web 2.0 generally refers to a second generation of services available on the World Wide Web that lets people collaborate and share information online. In contrast to the first generation, Web 2.0 gives users an experience closer to desktop applications than the traditional static Web pages. The term was popularized by O'Reilly Media and MediaLive International as the name for a series of web development conferences that started in October 2004. Web 2.0 applications often use a combination of techniques devised in the late 1990s, including public web service APIs (dating from 1998), Ajax (1998), and web syndication (1997). They often allow for mass publishing (web-based social software). The term may include blogs and wikis. To some extent Web 2.0 is a buzzword, incorporating whatever is newly popular on the Web (such as tags and podcasts), and its meaning is still in flux. O'Reilly recently claimed exclusive use of the term for conference names, further muddying the waters.
http://en.wikipedia.org/wiki/Web_2.0