According to wikipedia the phrase "Web 2.0" is one or more of the following:

• The transition of Web sites from isolated information silos to sources of content and functionality, thus becoming computing platforms serving web applications to end-users
• A social phenomenon embracing an approach to generating and distributing Web content itself, characterized by open communication, decentralization of authority, freedom to share and re-use, and "the market as a conversation"
• Enhanced organization and categorization of content, emphasizing deeplinking