(a) Ring and pinion gears are used to spin wheels and axles forwards and backwards, with their most common usage in motor vehicles to improve drive-train performance and provide extra power to the wheels.

(b) A transmission allows power to be sent from a power source, most often an engine or motor, to a drive mechanism. It uses gears and a clutch to convert the speed of the power source into torque. A simpler one is often referred to as a gearbox since it is basically a box containing a configuration of gears.

(c) A differential is a particular type of simple planetary gear train that has the property that the angular velocity of its carrier is the average of the angular velocities of its sun and annular gears.