A flywheel is used to store energy. When torque is applied to a flywheel, its energy is conserved in its rotational momentum. When the energy is needed, the energy is harvested and the wheel slows down.
The rotation of a flywheel slows down only slightly over time. A spinning top is a good example of how rotation stays relatively constant. Early flywheels were limited by their mechanical construction, but modern flywheels, which use magnets, can rotate tens of thousands of times per minute.
Early flywheels were used as pottery wheels, and they were later used in steam engines. Modern flywheels are often used to generate mechanical and electrical energy, and some are exploring using flywheels for hybrid automobiles. Others are exploring using flywheels as a means of storing and distributing solar and wind energy when the sun sets or when the wind is not blowing. Flywheels are cheaper, more dependable and, in some cases, more efficient than batteries at storing energy and generating electricity.
Flywheels have a number of additional benefits over batteries. They last for much longer periods of time, and they are not affected by temperature as much. In addition, flywheels are fairly simple devices, and the mechanics behind them are well understood.