In Starfleet, an enlisted or noncommissioned officer 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.
Any commissioned officer, from Ensign upward, directly outranks an enlisted officer in the chain of command. However, in certain cases, an enlisted officer can command commissioned officers (Miles O'Brien is one such case, in his position as Chief Operations Officer aboard Deep Space Nine).
Enlisted officers can apply to receive a commission, but must pass necessary examinations and/or attend the Academy.
As with commissioned officers, there is a rank structure amongst enlisted crew. These include Crewman (with various classes), Chief Warrant/Petty Officer, and Master Chief Warrant/Petty Officer.
Its predecessor, the Earth Starfleet, also had enlisted ranks.
Other force structures, such as the Klingon Defense Force and the Bajoran Militia, have enlisted men in their ranks, but it is unclear as to how they differ from their standard officer ranks.
See also:List of Starfleet enlisted personnel