Administrators (also known as sysops or admins) on MediaWiki.org are users with the technical capability to protect and delete pages, block users, and edit protected pages, among other things.
To become an admin, ask at Project:Requests. Information on the promotion process can be found there.
Stewards and global sysops should feel free to deal with vandalism and similarly routine and uncontroversial things, if necessary (Note: These are global groups defined by the CentralAuth extension and don't appear at Special:ListUsers, but at Special:GlobalUsers). If a task requires administrator tools, the community seems to agree that there should be no problem with this if it is not complex.