MediaWiki is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
Copyright 2001 - 2017. A list of authors can be found here.
Extensions and skins may be released under different licenses than MediaWiki itself. Most of them are usually distributed with a copy of their respective licenses and copyright notices.