MediaWiki - wiki software
An extremely powerful, scalable software and a feature-rich wiki implementation
Pages use MediaWiki's wikitext format, so that users without knowledge of XHTML or CSS can edit them easily
MediaWiki has an active volunteer community for development and maintenance
MediaWiki is free server-based software which 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, scalable software and a feature-rich wiki implementation, that uses PHP to process and display data stored in a database, such as MySQL.
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.
MediaWiki is wiki software
If you don't know what a wiki is, then read this Wikipedia article before going any further!
MediaWiki is server software
As with any software that you expose to the internet, there may be bugs or security problems. Do not install MediaWiki unless you intend to keep up with security upgrades (please subscribe to receive announcements of security updates).
MediaWiki is geared towards the needs of the Wikimedia Foundation
The program is primarily developed to run on a large server farm for Wikipedia and its sister projects. Features, performance, configurability, ease-of-use, etc are designed in this light; if your needs are radically different the software might not be appropriate for you.
MediaWiki is free software
No guarantee or warranty of any kind is provided.