The Wikimedia Foundation, Inc. is a non-profit charitable organization headquartered in San Francisco, California, United States, and organized under the laws of the state of Florida, where it was initially based. It operates several online collaborative wiki projects including Wikipedia, Wiktionary, Wikiquote, Wikibooks (including Wikijunior), Wikisource, Wikimedia Commons, Wikispecies, Wikinews, Wikiversity, Wikimedia Incubator and Meta-Wiki. Its flagship project, the English-language Wikipedia, ranks among the top ten most-visited websites worldwide.

for more information see:
http://en.wikipedia.org/wiki/Wikimedia_Foundation

This is one of many Wikipedia domains. While it is a good source of information the reliability is not always good. For any serious work double check your facts.

The Wikimedia Foundation is a non-profit organization. It operates numerous online community collaborative wiki projects such as Wikipedia (encyclopedia), Wikitionary (dictionary), and much more. It also allows other companies and sites to host their own Wiki based on their source codes. Excellent and trustworthy organization.