Simply put, Web 2.0 is the social web. It allows people to interact with and create web-based content in real time without purchasing software or owning servers.
Web 2.0 applications first emerged after the dot-com bubble burst in 2001. The term "Web 2.0" originated at O'Reilly Publishers in 2004. The hallmark of these applications is that they are in "perpetual beta." You can read more about perpetual beta in Ray
Schroeder's Sloan-C View article.