Auto Air compressor —The air compressor is the heart of the a/c system.  Refrigerant (Freon) is compressed and then pumped out to the other components of the a/c system.  A belt turns the compressor clutch on the front on the compressor. 

When the a/c switch is turned on inside the car, the clutch engages the compressor assembly and pumps the Freon.  The compressor is separate from the clutch assembly.