Web 2.0 is all about social interactions online. From comments, to blogs, to social networks of all sizes web 2.0 means interacting not just passively surfing and consuming information.
Here's an article I have bookmarked for reference on web 2.0, which I think presents a great vision of where the future development of the web is going.
It's on Richard Rosenblatt, the guy who sold Myspace to Fox and has since moved on the build a new company.