Different users have access to different functions of the site. While anyone can do most things on the site, including reading and editing, staff members can access a few additional functions.
Bureaucrats
A Bureaucrat has all the powers of an Administrator, with the addition of being able to make other users into bureaucrats or administrators on their wiki. Bureaucrats can also give users the ability to "rollback" edits not made in good faith.
List of Bureaucrats
Administrators
Administrators can delete and undelete pages, page histories and uploaded files. They can lock (protecting) a page so it cannot be edited or renamed by users without admin rights or autoconfirmed status and block IP addresses or user names from editing; and very quickly "rollback" undesirable edits. They can also edit the interface by changing system messages and skins.
List of Administrators
Content Moderators
In addition to rights and responsibilities of a Rollback, Content Moderators can also protect and delete articles and images to combat vandalism, prevent edit warring and remove spoilers.