The small reaction engines a starship uses for slow, precise movement when docking, leaving port or holding position, where impulse or warp power would be unsafe.
Thrusters are the low-powered engines spaced around a starship's hull. They turn the ship and move it slowly and exactly, and they are what a helm officer uses near a station, another vessel or a dock.
Starfleet rules restrict ships to thrusters inside Spacedock. In Star Trek VI: The Undiscovered Country, Lieutenant Valeris reminds Kirk of this as the Enterprise departs, and he orders one quarter impulse power anyway, leaving her to fly the ship out through the doors.
Thrusters can also be all a ship has left. In Booby Trap the Enterprise-D is caught in an ancient field that drains power the more the engines are used, and Picard takes the helm himself, shuts almost everything down and brings the ship out with a couple of short thruster bursts and the pull of a large asteroid.