From UFStarfleet LCARS
In United Federation Starfleet, an enlisted is a person who did not attend Starfleet Academy. In most cases enlisted crewman only recieve basic training as well as job specific courses required for their position.
The enlisted rank structure is called 'GRADES' for the United Federation Starfleet Marine Corps and 'RATES' for their fleet counterparts.
This also includes non-commissioned officers. the term NCO refers to an enlisted who has authority over other personnel. Although any enlisted is outranked by a simple ensign, it is not unusual that a Non- Commissioned Officer has certain authoritity over commissioned officers