Developers are the people contributing to the code of MediaWiki software. They commit changes to the central Git repository by pushing changes for review in Gerrit where the latest copy of the software is stored. Developers include paid Wikimedia Foundation staff and volunteers. Anyone can submit a patch for review after creating a Developer account (but only maintainers can merge it). For statistics or names, see Development statistics.
Developers are not to be confused with system administrators, who are people having shell or root access to Wikimedia Foundation servers, where the code repository is stored. They might not really be developers; in many cases they rarely use their commit access, or if they do, only to maintain non-MediaWiki things in the repository.
A maintainer of a specific project (including MediaWiki core or an extension) has +2 access to the relevant Git repository, so that they can merge patches others submitted. They usually regularly respond to bug reports and changeset review requests. A list of maintainers are kept at Developers/Maintainers.
Historically, developers also managed Wikimedia Foundation servers (now managed by system administrators). Before bureaucrat and steward groups were created, developers were the only ones able to promote and demote sysops, and lock user accounts (before the "block" feature of MediaWiki existed). Thus they had an important role in the Wikimedia power structure.
From April 2006 to March 2012, Subversion was used. Only people with Subversion commit access (which must be approved by SVN administrators) could submit code changes. Code changes submitted to Subversion was merged to MediaWiki codebase immediately (though they could only be manually deployed to Wikimedia wikis by a system administrator), and may have been reviewed by "coders" before they are deployed. In January 2013, the MediaWiki SVN repository was made read-only. All active projects have moved to Git.