Clutch: 

A device used to engage and disengage the engine to the power train or gearbox without any jerk. There are types like single plate and multiplate clutches where the power is transmitted through pressure and friction plates. Centrifugal clutch is of another type which engages by the centrifugal force.

Flywheel: 

A device used to control the fluctuations of energy. It stores the energy whenever the engine power production is more than the demand and vice versa. 

Disc type and arm type are widely used designs.

To maintain constant angular velocity of crankshaft which transmits the power to the clutch and powertrain it is necessary to control the fluctuations.