Wikipedia is a free online encyclopedia. Instead of being authored by a select group of editors, Wikipedia can be edited by anyone anywhere, and at any time. Wikipedia is available in a large number of languages and has entries about nearly everything. It's become so well known, people consider it an adjunct to Google as a place to find out more about almost any subject.
Each Wikipedia entry gets its own article as well as a built-in discussion in which users can talk about things they want to add or request changes from frequent contributors. There's also a history page that tracks every single change made to the article, as well as a means of reverting back to previous versions in case of disaster or user vandalism.
While Wikipedia pages are frequently the victims of vandalism and inaccuracies, there is a large base of roving editors who do their best to maintain civility and focus.