The job of a cars AC system is to transfer unwanted heat from inside the car and transport it outside. Refrigerant goes through the system to the Evaporator in liquid form. Air is blown across the evaporator and the refrigerant inside heats into a vapor, absorbing the heat in the process. After the refrigerant is vaporized it is transported to the Condenser where it is condensed back to a liquid by releasing the heat back to the outside air. (By ram effect of driving or through implementating a cooling fan). The refrigerant is then returned to the evaporator where it is heated back to a vapor where the cycles begins again.
The evaporization and condensation of the refrigerant are triggered by creating a low pressure zone in the system where the refrigerant can easily vaporize and condense.
The Compressor in the system serves as the pump to draw low pressure refrigerant in vapor form from the evaporator. Inside the compressor the refrigerant is compressed into a high pressure vapor which is pushed to the condenser to be cooled back to a liquid.
A control valve, in our case an Orifice Tube at the inlet to the evaporator meters the correct amount of refrigerant to be sent into the evaporator. This orifice tube is one of the zones that separates the high and low pressure areas of a system. The other area is the Compressor where low pressure vapor is turned to high pressure vapor.
Any other components in an Air Conditioning system are there primarily to boost efficiency, control, or safety. Most systems utilize a dryer to remove unwanted moisture from inside the system which can cause poor cooling due to icing of the metering device, or rust of the ferrous parts of the system, particularly inside the compressor.