A Web 2.0 site may allow users to interact and collaborate with each other in a social media dialogue as creators of user-generated content in a virtual community, in contrast to Web sites where people are limited to the passive viewing of content. Examples of Web 2.0 include social networking sites, blogs, wikis, folksonomies, video sharing sites, hosted services, Web applications, and mashups. (Wikipedia)
Formerly known as World Wide Web 1.0 when everything was designed in one way, Web 2.0 introduced the two ways of communicating. Examples given above are the clear picture of what Web 2.0 is all about. The instant example is Facebook whereas users can communicate with each other, notification to let them know of their friends responded to their posts, and tons more.
It somehow improved a lot more and making the World Wide Web a more interesting place to visit. Socializing-wise.