Web 2.0 isn't a technology, it's a concept: enhance the software (programs) with wetware (humans). WikiPedia, YouTube, Flickr and the like are thriving because the users are part of the equation; creating, editing, organizing and managing the content.

Michael Wesch, a professor of cultural anthropology at KSU created a fantastic video which artfully illustrates Web 2.0.  It's a fantastic romp through today's digital landscape... but I'll warn today's TV generation... some reading is required.

 Via information aesthetics