Gearboxes/Speed Reducers work to either reduce the high speed and lower the torque or increase the speed of the prime mover output shaft to allow higher forces to increase torque. to.  A gearbox consist of bearings, gears and shafts that are enclosed in a factory mounted housing.  They can either be mounted directly on a motor or just the shaft. The input and output configurations vary from solid shaft, hollow shaft, coupled or bushing. Special tip when ordering would be to know the gear ratio, output torque, maximum input power and input speed.