Administrators (also known as sysops) are users with the technical ability to:
delete and undelete pages, and view deleted revisions of pages; block and unblock users, individual IP addresses, and ranges of IP addresses; protect and unprotect pages, and edit protected pages; edit the interface (MediaWiki namespace) and user configuration files; import pages from other Wikipedias; perform other functions related to the technical maintenance; add and remove users from some usergroups like rollback, IP block exempt or flooder.
They can only perform these actions locally, on the wiki that they have been given adminship.
The role and expectations of administrators, as well as the policies surrounding their behavior, vary widely among the different projects. While administrators have more technical abilities than ordinary users, this should not be confused with authority, which is earned separately and given by the community. Some wiki sites may not even have adopted written policies for administrators or candidates for adminship. This is usually due to limited number of users and activities.