An expendable launch system throws away the rocket in order to launch some object, called the "payloadUsually the goal is to get the load into low Earth orbit.
Exendable launchers have been simple modifications of 1950's era balistic missiles. Since the entire vehicle is discarded after launch, they are obviously an expensive launch method. The military organizations maintain expendable launchers because they are perceived as a having a low risk of mission failure, and a short time to launch.
See also space transportation and spacecraft propulsion.