Wikipedia is a collaboratively edited, multilingual, free Internet encyclopedia that is supported by the non-profit Wikimedia Foundation. Anyone who can access the site can edit almost any of its articles.
As with any publicly edited and created content website you must use caution and verify anything you see as suspicious or untrue.
Part of wikipedia.org MediaWiki is a free software open source wiki package written in PHP, originally for use on Wikipedia. It is now also used by several other projects of the non-profit Wikimedia Foundation and by many other wikis.