Rocket Motors burn solid grain propellant to generate large amounts of thrust.  Launch vehicle designers typically have to decide between using liquid rocket engines versus solid rocket motors.  Rocket motors have more thrust, but are less efficient than liquid rocket engines.  Furthermore, the rocket motor's dense solid propellant make them more compact.  As a result, rocket motors are typically used as strap-on boosters for first stages, such as the Space Shuttle, and for applications that require simple compact designs.