This site references O'reilly a lot, as he's apparently the authority on all things Web 2.0. There are many competing definitions for Web 2.0. Some say it's a set of practices and philosophies that help users have a deep, rich web experience. Others think it's a new collection of technology that increases accessibility for everyone. Still others think it's all hype and it means nothing...
This article as a whole defines Web 2.0 technologies and I like that it includes examples of what Web 2.0 technologies are. This article could be used in a writing classroom to encourage a dicussion aobut what it means to compose in the twenty-first century.
This is a very apporpriate definition. Web 2.0 is simply a new way to use the internet. If we look at the definiton of Web 1.0 it is simply the use of search engines, and email, etc. However, web 2.0 takes these basic tasks to the next level. I like to think of web 2.0 as a function of new media as well.