A retrorocket is a rocket engine that is fired to slow the vehicle on which it is mounted. Applications include causing an orbital or suborbital vehicle to reenter the atmosphere, or, especially in the absence of atmosphere, braking a space vehicle to a "soft" landing. Some have been used to brake the landing of an airdropped object, with and without parachutes.
Some are liquid fuel rockets, especially when it is necessary to vary thrust; the rockets often use monopropellants or hypergolic propellants for mechanical simplicity. Solid fuel rockets, however, are widely used when the braking requirements are well understood and the rocket can be engineered to match that requirement.
A thruster is a more general case of a retrorocket, in that retrorockets are primarily used to provide thrust opposing the primary flight path, where thrusters can provide counterthrust in many directions.