Bureaucrats are users that have the powers of sysops, as well as the ability to promote users. They are generally considered the head administrators of the site. Bureaucrats as well as sysops usually have colored names and avatar borders. The maximum amount of bureaucrats is currently three.
Chat moderators are essentially administrators on chat. They can kick and ban people from chat. Their powers do not apply to the wiki, though, unless they are a sysop. Reverters are users that have the ability to rollback pages with a special button to easily undo edits.