A clutch is a device for controlling the
torque applied to a
mechanical system.
In its simplest form, its two plates with high-friction surfaces on them. One plate is connected to a source of torque, such as a motor. When the plates are forced together mechanically, it will cause the second plate to start spinning. The second plate is generally connected to a drive shaft of some description used to apply the force to a mechanical system.
There are also other types of clutches. The one-way clutch, much like it sounds, only allows torque to be transmitted in one direction. Most one-way clutches resemble ratchets. The electro-mechanical clutch is a simple clutch, with one addition...an electromagnet is used to force the plates together. A slip clutch is a form of torque limiter, its used to prevent an overload. It usually consists of a tight spring wound around an arbor, such that it can slide if too much torque is applied.