Web 2.0 (2005) is the term given to describe a second generation of the World Wide Web that is focused on the ability for people and businesses to collaborate and share information online.

Examples of Web 2.0 include:
Social networking sites
blogs
Wikis
Customer to Customer (classifieds, auctions)

Reference:
Top 15 Most Popular Social Networking Sites
What is Web 2.0? - A Word Definition From the Webopedia Computer Dictionary
Web 2.0 - Wikipedia, the free encyclopedia
Web 2.0 Events: Co-produced by UBM TechWeb and O'Reilly Conferences