MediaWiki is free and open-source wiki software, originally developed by Magnus Manske and improved by Lee Daniel Crocker.
MediaWiki runs on many websites, including Wikipedia, Wiktionary and Wikimedia Commons. It is written in the PHP programming language and stores the contents into a database. Like WordPress, which is based on a similar licensing and architecture, it has become the dominant software in its category. Its development has since then been coordinated by the Wikimedia Foundation.
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 is able to support such requests.
Because MediaWiki is flexible, many websites that want people to contribute information use MediaWiki rather than other types of wiki software.
MediaWiki provides a rich core feature set, which include:
Internationalization and localisation
Installation and configuration
Editing Interface (and Editing in Wiki Markup language)
Rich content functionalities
Content organization (pages, sub-pages, tags, categories etc )
Customization and Templates
For further reading on the full features list, you can check here