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. As used by its proponents, the phrase refers to one or more of the following:

• The 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, characterized 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 far more developed deep linking web architecture
• A shift in economic value of the web, possibly surpassing that of the dot com boom of the late 1990s
• A marketing term to differentiate new web businesses from those of the dot com boom, which due to the bust now seem discredited

Check it out here…

http://en.wikipedia.org/wiki/Web_2.0 

http://web2.wsj2.com/web2ishere.htm