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. The articles that have been most susceptible to falsified statements are biographies of living persons (BLP). As such, Wikipedia recently proposed a plan that would require editor approval before changes to such articles could take effect. Whether this will change the overall spirit of the service going forward remains to be seen. As for now, it remains a wealth of information, most of it entirely reliable.