From Bravo Fleet Infobase
A non-commissioned officer (sometimes noncommissioned officer), also known as an NCO or Noncom, is an enlisted member of Starfleet who has been given authority by a commissioned officer. The NCO corps includes all the grades of sergeant and, corporals and warrant officers. The Starfleet equivalent includes all grades of petty officer.
The non-commissioned officer corps is the "junior" management of the military. They serve as administrative or training personnel and as advisors to the officer corps. However, their most valuable function is as a link between the 'common soldier' and the (commissioned) officer, who are encouraged to not develop a too close personal relationship with his charges. Because officers - other than those who join as enlisted soldiers and are later commissioned - begin their careers in a position of authority but lack practical experience, senior NCOs are also often responsible for giving them on-the-job training.