Bureaucrats are administrators who have extra roles on top of the Administrator role. They are the highest role obtainable by any user.
Rights of bureaucrats
Bureaucrats have the ability to grant and revoke the moderator and administrator roles, as well as appointing new bureaucrats (although the bureaucrat role cannot be revoked by other bureaucrats).
Aside from these, bureaucrats have no additional rights, so the bureaucrat right is given on top of the administrator right.