Web 2.0 site usually fits into one of the following types:

• Community as the content - social networking: members share information, the community decides the value of that information and the community benefits. A two-way open sharing of information.See: The Cluetrain Manifesto, Web 2.0 diagram, Oreilly Web 2.0 Article.
  Examples: Digg (share tech links), 43 Things (share to do lists) and del.icio.us (share bookmarks).
• Rich online applications - easy-to-use applications which take software which may have once been found on single desktops to a shared online space, independent of the computer you are using. Examples: Flickr (photos), Writely (Word processor) and Backpack (organiser).

[Source: cre8d-design]