A clutch is a mechanical device which engages and disengages power transmission especially from driving shaft to driven shaft.In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts).
source/image(PrtSc): Simon Raisbeck
A basic clutch is made up of three major parts: Flywheel, Friction disk, Pressure plate.The flywheel is attached to the back of the engine. The pressure plate is attached to the flywheel.
Sandwiched in between the two is the friction disk. The friction works a lot like brake pads, in that it grips the flywheel/pressure plate on either side through, what else, friction.
The clutch connects the two shafts so they may be locked together and spin at the same speed (engaged), locked together but spinning at different speeds (slipping), or unlocked and spinning at different speeds (disengaged).