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). In these devices, one shaft is typically attached to an engine or other power unit (the driving member) while the other shaft (the driven member) provides output power for work.
Below are a couple great YouTube Videos, demonstrating the components of a motorcycle clutch, how they go together, and how the clutch operates / exactly what it does.