MediaWiki is a free software package originally written for Wikipedia but is now run on other projects of the non-profit Wikimedia Foundation and many other wikis. (from MediaWiki)
“MediaWiki is a free server-based software, that is licensed under the GNU General Public License (GPL). It’s designed to be run on a large server farm for a website that gets millions of hits per day. MediaWiki is an extremely powerful, scaleable software and a feature-rich wiki implementation, that uses PHP to process and display data stored in its MySQL database. Pages use MediaWiki’s wikitext format, so that users without knowledge of XHTML or CSS can edit them easily. When a user submits an edit to a page, MediaWiki writes it to the database, but without deleting the previous versions of the page, thus allowing easy reverts in case of vandalism or spamming. MediaWiki can manage image and multimedia files, too, which are stored in the filesystem. For large wikis with lots of users, MediaWiki supports caching and can be easily coupled with Squid proxy server software.” (from MediaWiki’s “How does MediaWiki Work?”)
MediaWiki should not be confused with Wikimedia: “Wikimedia is the collective name for a group of inter-related projects, including Wikipedia, Wiktionary, Wikisource, Wikibooks, and others, which aim to use the collaborative power of the Internet, and the wiki concept, to create and share free knowledge of all kinds. Like “Wikipedia”, “Wikimedia” is a trademark, and should not be used for projects which are not officially affiliated. Wikimedia is also used as a shortened form of The Wikimedia Foundation.” (from MediaWiki’s “Names” page)
Starter Links: MediaWiki Homepage | List of Sites that use MediaWiki