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Wikipedia dicit:

A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control, and sometimes translation. Use of diverted engine thrust to provide stable attitude control of a short-or-vertical takeoff and landing aircraft below conventional winged flight speeds, such as with the Harrier “jump jet”, may also be referred to as a reaction control system.

An RCS is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw).

Reaction control systems often use combinations of large and small (vernier) thrusters, to allow different levels of response. Spacecraft reaction control systems are used for: attitude control during re-entry, stationkeeping in orbit, close maneuvering during docking procedures, control of orientation, or ‘pointing the nose’ of the craft, a backup means of deorbiting, ullage motors to prime the fuel system for a main engine burn.

Because spacecraft only contain a finite amount of fuel and there is little chance to refill them, alternative reaction control systems have been developed so that fuel can be conserved. For stationkeeping, some spacecraft (particularly those in geosynchronous orbit) use high-specific impulse engines such as arcjets, ion thrusters, or Hall effect thrusters. To control orientation, a few spacecraft, including the ISS, use momentum wheels which spin to control rotational rates on the vehicle.

Video Credit: Copenhagen Suborbitals