MEDIA WIKI

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. 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
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.

What MediaWiki is...

MediaWiki is wiki software.
MediaWiki is server software.
MediaWiki is geared towards the needs of the Wikimedia Foundation.
MediaWiki is free software.

Some things MediaWiki doesn't do so well...

•    Because MediaWiki was designed for open-content, it is often not suitable for situations where you want to restrict access to part of the wiki.
•    MediaWiki has been designed to serve very high-traffic websites such as Wikipedia. It has been optimised for this use and may not be so suitable for smaller sites, where disk space or memory are bigger restrictions than bandwidth.
•    MediaWiki is not normally a suitable replacement for dedicated forum/blogging software if that is what you are after.
In the above cases there may be other wiki software (or non-wiki software) which better serves your needs.
Whether you should use MediaWiki for your own wiki depends on what kind of website you want to run with it. Wikipedia's software has many useful features that grew out of the needs of the users and the ideas of the developers -- but features also mean complexity.

Licence

MediaWiki is free software licensed under version 2 of the GNU General Public License. Because MediaWiki is licensed free of charge, there is no warranty, to the extent permitted by applicable law. Read the full text of the GNU GPL version 2 for details.

System Requirements

MediaWiki requires PHP 5.3.2+ and either MySQL 5.0.2+, PostgreSQL, or SQLite. For more information, please read the system requirements andcompatibility.
Note: To get a suitable version of PHP on CentOS 5.6, install the php53 package instead of php.
To MediaWiki 1.18.x users: Support of this branch will be discontinued at the end of November 2012. Please prepare to update to a newer version of MediaWiki:
•    MediaWiki 1.20.0 (download)
•    MediaWiki 1.19.2 (download)

Twitter/Facebook/Identi.ca
We have an official Twitter account.
•    @MediaWikiOrg
And Facebook page also.
•    MediaWiki
To hear about upcoming events and meetings, subscribe to our meetings account.
•    @MediaWikiMeet (Twitter)
•    @MediaWikiMeet (Identi.ca)
Blog
The Wikimedia Blog has a tech section specifically for news and information from the Wikimedia Foundation’s Technology department (link - RSS feed).
Newsletter
English Wikipedia's Signpost has a regular technology report with content related to activity within Wikimedia Foundation’s Technology department.
Websites
•    Wikimedia's Meta-Wiki was formerly where documents were managed and proposals were discussed before this site was started. There is still a lot of content there that has yet to be moved.
•    MediaZilla is where feature requests and bugs may be reported or browsed.