Wikipedia is a web-based and collaborative encyclopedia project.
As of January 2012, Wikipedia has 20 million articles — more than 3.8 million of them in English — which have been written by volunteers from around the world. Almost all of its articles can be edited by anyone with access to the site. Wikipedia’s tagline is “the free encyclopedia.”
Wikipedia was launched in 2001 by Jimmy Wales and Larry Sanger and has become the largest and most popular general reference work on the Internet. It began as a complement to Nupedia, an online encyclopedia project (its tagline was also “the free encyclopedia”) whose articles were written by experts — generally academics — and reviewed under a formal process. (While Wales is credited with the general idea of making a publicly editable encyclopedia, Sanger is usually credited the idea to use a wiki to reach that goal.)
Wikipedia was formally launched on January 15, 2001 through an announcement Sanger made to the Nupedia mailing list. There were initially relatively few rules about writing and editing articles, though Wikipedia’s “neutral point-of-view” policy — the ethos that guides its editorial process — developed quickly among its contributors.