The second phase of architecture and application development for the web. 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 has become a buzzword, incorporating whatever is newly popular on the Web (such as tags and podcasts). A consensus on its exact meaning has not yet been reached. Proponents of the Web 2.0 approach believe that Web usage is increasingly oriented toward interaction and rudimentary social networks, which can serve content that exploits network effects with or without creating a visual, interactive web page.
Man, who knows for sure? This is a world class buzzword. Mostly, it refers to advanced Web design, but there's also a political element too, with regard to the ways companies interact with their users. Tim O'Reilly coined the term, so if you want to know more, start out with his short and long descriptions.
A term applied to the transition of the Internet from a collection of connected websites to a computing platform with rich web applications. Also applied to sites which exhibit the rich web characteristics of the transition.