AC compressor is commonly known as the heart of the system, the ac compressor is a belt driven pump that is fastened to the engine. It is responsible for the compressing and transferring refrigerant gas. The AC system is split into two sides, a high pressure side and a low pressure side, defined as discharge and suction.
The ac compressor compresses Freon into a high pressure, high temperature gas which is then sent to the condenser for the process of removing the heat. The fact that this temperature is greater than the outside temperature allows the heat to transfer to the outside.
The compressor typically contains the Compress or Clutch as well. If you are looking for the Clutch only you would need to purchase the entire assembly.
AC Compressor is inspected for leaks.
Freon or coolant.
The AC Compressor is the part that holds a charge of freon that cools the car. It usually includes a clutch as seen in the left hand side of this picture.