This category covers sites and content defining, describing, and commenting on "Web 2.0" as a topic. It does not deal with any "Web 2.0" sites themselves.
The "Web 2.0" concept was first used by O'Reilly Media in 2004. It refers to a perceived or proposed second generation of web-based services and sites that include collaboration and sharing among users, as contrasted with early (late 1990's) web sites' emphasis on content consumption by visitors. Examples include social networking sites, wikis, folksonomies, communication tools, and other web applications offering a rich user experience.
Note that neither examples of "web 2.0" sites nor those giving technical implementation details belong in this category; please find more appropriate categories for such sites. Suggesting off-topic sites to this category will only delay their review and potential listing.