Web 2.0 is an expression that's made its way around the Internet the past few years and despite the amount that it's used, few people really understand what it means.
Web 2.0 generally refers to websites that encourage some sort of user interaction. What kind of interaction and how necessary that interaction is to the site seems not to matter, but when it comes to driving traffic, interaction is one of the best things that have ever come out of the internet.