What is Web 2.0?
Web 2.0 is NOT any one technology, it is an abstract concept, much like "pretty" or "pale". We use this term to embody a set of new ideas that gaining popularity on the Internet. These ideas are very different from old ideas and current practices.
Web 2.0 is a service, not a product. It is webmail, blogs, web-based word processing. It is a tool that you use over the Internet, not on your local machine.
Web 2.0 is about the user experience. It is dynamic HTML (and perhaps Flash), to allow for a rich user experience. For example, the user types in their zip code and their city appears without refreshing the page.
Web 2.0 is about trusting the user. It is Wikipedia, allowing the user to update the site content. It is Digg.com, a news site completely driven by user submitted stories.
Web 2.0 is about the user contributing to the content. Bookmark (social tagging) sites allow users to bookmark links, and add keywords to them, which is then used by other users to search on, this is Web 2.0.
Web 2.0 is about the "long tail". It is about not targeting a small group, but as wide a market as possible. Music stores stock only the best sellers, appealing to a core audience. In contrast, iTunes has over 2 million songs available, appealing to a much wider market. It has been said that Amazon less best-sellers than it sells less popular books due to the sheer quantity of book available. Web 2.0 is about appealing to everyone, and not a specific audience.
Who is Web 2.0?
I think this description is a good start, but I may need to tweak it a bit. If you disagree with me, or think I missed something important, drop me a comment.