Web 2.0 , a phrase coined by O'Reilly Media in 2004, refers to a supposed second-generation of Internet-based services — such as social networking sites, wikis, communication tools, and folksonomies — that let people collaborate and share information online in ways previously unavailable. O'Reilly Media, in collaboration with MediaLive International, used the phrase as a title for a series of conferences and since 2004 it has become a popular (though ill-defined and often criticized) buzzword amongst certain technical and marketing communities.

Alluding to the version-numbers that commonly designate software upgrades, the phrase "Web 2.0" hints at an improved form of the World Wide Web, and some people have used the term for several years.

In the opening talk of the first Web 2.0 conference, Tim O'Reilly and John Battelle summarized key principles they believed characterized Web 2.0 applications:

Commentators see many recently-developed concepts and technologies as contributing to Web 2.0, including weblogs, linklogs, wikis, podcasts, RSS feeds and other forms of many-to-many publishing; social software, web APIs, web standards, online web services, and others.

Proponents of the Web 2.0 concept say that it differs from early web development (retrospectively labeled Web 1.0 ) in that it moves away from static websites, the use of search engines, and surfing from one website to the next, towards a more dynamic and interactive World Wide Web. Web 2.0 sites act more as points of presence, or user-dependent web portals, than as traditional websites