Refrigerant exits the compressor (A) in a gaseous form under high pressure (125 - 250 psi) and travels to the condensor (B) which is located in front of the car radiator. The incoming air cools the refrigerant which turns it into a liquid.
It goes into the accumulator (receiver/dryer on some systems) (C) where remaining gas bubbles are separated and the refrigerant is filtered. The liquid travels to an expansion valve or orifice (D) which greatly reduces the flow causing a low pressure (usually 20 - 40 psi). This is known as the suction side of system. It immediately enters the evaporator (E), the unit under the dash.
With the sudden pressure drop in pressure the liquid is very cold and as the incoming air flows through the evaporator it removes the heat from the air before it gets to the passenger compartment. The refrigerant is now warm when it leaves the evaporator and has turned back into a gas where it is sucked back into the compressor for the return trip.
The pressure on the high side is determined by the amount of air flowing past the condenser and the temperature of the air going over the condenser and through the evaporator. Poor air flow can cause too high of pressures which results in poor A/C operation and possible compressor damage.
The above system is very basic and does not show the safety switches and controls or many of the electronics found on