Web 2.0 is a concept rather than a particular technology or architecture. Web 2.0 refers to the idea of the Internet and websites that make up the world wide web being interactive vs one-sided.
Prior to Web 2.0 websites were essentially passive, meaning they were built for people to look at and read information. Today the majority of websites on the Internet are interactive and allow communication and dialog among their users. This is the core difference between the Internet before 2.0 and after.
The term “Web 2.0″ was first used in January 1999 by Darcy DiNucci, DiNucci writes:
The Web we know now, which loads into a browser window in essentially static screenfuls, is only an embryo of the Web to come. The first glimmerings of Web 2.0 are beginning to appear, and we are just starting to see how that embryo might develop. The Web will be understood not as screenfuls of text and graphics but as a transport mechanism, the ether through which interactivity happens. It will [...] appear on your computer screen, [...] on your TV set [...] your car dashboard [...] your cell phone [...] hand-held game machines [...] maybe even your microwave oven.
Top Web 2.0 Regional Websites
Top Web 2.0 User Generated Content Websites
The ideas behind Web 2.0 really took off with the rapid growth of social media websites. One could argue that a Web 2.0 website is a social media website, but a better statement would be that a social media website is an example of Web 2.0 at work.