Clutch systems are used in gear systems to allow the power to the gear box to be seperated from the drive of the motor or engine. 

This allows a gear box to shift between gears without dammagng the gears / cogs  teeth of the gears. 

The clutch is useually found between the input shaft of the gearbox and the output shaft / gear of the engine or motor. 

Applying pressure to the clutch pedal compresses the pressure plate springs contracting it so that it releases the clutch disk from the flywheel. This seperates the action of the drive from the motor or engine to the gearbox allowing for smooth gear changes without crunching or breaking the gears. 

Looking closely at the diagram on the right you can see that the pressure plate (yellow) is pressing firmly into the flywheel causing the clutch to engage the drive power from the engine to motor with the input shaft of the gearbox.  

This is how a clutch would look if the clutch pedal was not being pressed. 

This is a diagram of the clutch in a disengaged position, if you look closely you will see that the pressure plate (yellow) has a gap between it and the flywheel. 

This is how a clutch would look when the clutch pedal / lever is depressed.