A thruster is a propulsive device used by spacecraft for station keeping, attitude control, in the reaction control system, or long-duration, low-thrust acceleration. A vernier engine or gimbal engine is a particular case used on launch vehicles where a secondary rocket or other high thrust device is used to control the attitude of the rocket while the primary thrust engine (generally also a rocket engine) is fixed to the rocket and supplies the principal amount of thrust.
Some devices that are use or proposed to use as thrusters are:
Rocket engine, using exothermic chemical reactions of the propellant(s)
Electrohydrodynamic thruster, using ionized air (only for use in an atmosphere)
Electrostatic ion thruster, using high-voltage electrodes
Ion thruster, using beams of ions accelerated electrically
Hall effect thruster, a type of ion thruster
Pulsed inductive thruster, a pulsed form of ion thruster
Magnetoplasmadynamic thruster, electric propulsion using the Lorentz force
Electrodeless plasma thruster, electric propulsion using ponderomotive force
Pulsed plasma thruster, using current arced across a solid propellant
RF resonant cavity thruster, an electromagnetic thruster using microwaves
