A vehicle's clutch is a device for quickly and easily connecting or disconnecting a pair of rotatable coaxial shafts. Clutches are usually placed between the driving motor and the input shaft to a machine and provide a convenient means for starting and stopping the machine and permitting the driving motor or engine to be started in an unloaded state (as in an automobile). Mechanical clutches provide either a positive (no-slip) or a friction-dependent drive; centrifugal clutches provide automatic engagement. An overrunning clutch transmits torque in one direction only and permits the driven shaft of a machine to freewheel (continue rotating after the driver stops); on bicycles, such clutches permit the rider to coast without moving the pedals.

Furthermore, clutches permit the drive train of a machine to be connected to, or disconnected from, a prime source of power. It usually operates on a mechanical principle with friction surfaces that can be joined or separated, but other types include a fluid coupler.