In Starfleet, an enlisted crewperson is one who has not completed the four-year Starfleet Academy course. In most cases, enlisted crewmen sign up directly to a ship or posting, and receive basic training, as well as any specific courses required for their position.
In Starfleet, a “crewman" could refer to either a person who held the actual rank or simply any person who was assigned to an organization or starship. The grade of crewman could further be divided into several “classes”, such as “crewman first class”, “crewman second class”, etc.
All commissioned officer ranks, from ensign upward, outrank all enlisted in the chain of command. However, in certain cases, non-commissioned officers can exercise authority over commissioned officers. (Miles O’Brienwas one such case in his position as chief of operations aboard Deep Space 9.) Additionally, chief petty officerssuch as O’Brien wield influence far beyond their place in the rank structure due to their extensive experience and skill, and are expected to mentor junior officers as well as other non-commissioned officers.
Enlisted personnel may be promoted to receive a commission, but must pass necessary examinations and/or attend the Academy.