Wikipedia has an answer!
The term “Web 2.0” refers to what some people see as a second phase of development of the World Wide Web, including its architecture and its applications. It was coined by Dale Dougherty during a meeting between O’Reilly and Associates (a computer book publisher) and MediaLive International (an event organiser) as a marketable term for a series of conferences .
As used by its proponents, the phrase refers to one or more of the following:
- a transition of websites from isolated information silos to sources of content and functionality, thus becoming a computing platform serving web applications to end users
- a social phenomenon referring to an approach to creating and distributing Web content itself, characterised by open communication, decentralization of authority, freedom to share and re-use, and “the market as a conversation”
- a more organized and categorized content, with a more developed deeplinking web architecture.
- a shift in economic value of the web, potentially equalling that of the dot com boom of the late 1990s.