- The following is copied verbatum (minus the links) from the Introduction on the Wikipedia page on MediaWiki:
MediaWiki is a wiki software package licensed under the GNU General Public License. It is written in PHP and requires the MySQL relational database management system (although it has some PostgreSQL support). Historically, MediaWiki was developed to serve the needs of Wikipedia, a free wiki-based encyclopedia, but it has since become one of the most popular general wiki solutions. Today, it is used by all projects of the Wikimedia Foundation, all wikis hosted by Wikia, and many other popular and large wikis. It has also been deployed by companies as an internal knowledge management solution, and as a content management system. Notably, Novell uses it to operate several of its high traffic websites, which are not editable by the general public.
MediaWiki provides a rich core feature set and a mechanism to attach extensions to provide additional functionality. Due to the strong emphasis on multilinguality in the Wikimedia projects, internationalization has received significant attention by developers. The user interface has been fully or partially translated into more than 70 languages, and can be further customized by site administrators (the entire interface is editable through the wiki). Due to Wikipedia being one of the world's largest websites, achieving scalability through multiple layers of caching and database replication has also been a major concern for developers. Wikipedia and other Wikimedia projects continue to define a large part of the requirement set for MediaWiki.