Author Shiv Singh has a good summary of Web 2.0 and the Enterprise on Boxes and Arrows:

What is Web 2.0?
To paraphrase a definition by Tim O’Reilly, who was one of the first to use the term, “Web 2.0” is web-based software which is continually collaboratively updated. This means that the software gets more useful the more people who consume and remix it. Remixing is a key concept of Web 2.0. In music, remixing means taking established songs and editing them together, potentially adding your own elements as well. With Web 2.0, individual users add their own data and services to collaborative web software, remixing the Web 2.0 sites into increasingly useful tools and creating an exponential growth effect.