In military service, a noncommissioned officer (NCO) has responsibility for the training, readiness, and supervision of individuals. In contrast, commissioned officers have that responsibility for units.
Land and air forces NCOs are most often called sergeants, with various qualifiers indicating rank within their specialty. Naval NCOs are usually called petty officers.
Depending on the country and arm of service, there also are a variety of formal and informal traditional titles. Even though the formal rank of Gunnery Sergeant is not the highest rank of United States Marine Corps NCO, the informal and respectful term for the senior sergeant of a unit is "gunny". The formal term for certain senior Marine sergeants, in a supervisory rather than technical track, is First Sergeant or Sergeant Major.
In Commonwealth countries, certain functions that would be, for example, a First Sergeant in the U.S. may be called warrant officer. This can be quite confusing, because while warrant officers, in the US, are normally technical specialists between noncommissioned officers and commissioned officers, that is sometimes true of a British warrant officer (e.g., a naval boatswain), but not true of a Regimental Sergeant Major (i.e., job title) who has a warrant officer rank.