Web 2.0 is a coined term from Dale Dougherty during a meeting between O'Reilly and Associates (a computer book publisher) and MediaLive International (an event organiser) as a marketable term for a series of conferences.
The idea itself is a new wave of developmental projects throughout the web. Yahoo! Answers is an example of this. Web 1.0 was when webpages were static (non-changing). New webpages tend to be much more interactive. For a much more detailed look, see my source.
Some examples of Web 2.0 sites:
http://www.wiziq.com/
http://www.digg.com/
http://www.flikr.com/
http://www.writely.com/Default.aspx (an online word processor)
Source(s):
http://en.wikipedia.org/wiki/Web_2.0