Here's How Air Conditioning Works 
(in functional order; the components listed on the previous page): 

Follow closely, because these are the key steps to a functional air conditioning system. 

Compress, heat, cool, condense, dry, expand and cool, evaporate.

DETAILED OPERATION:

First lets start with the compressor. It's primary function is to compress and pressurize gaseous refrigerant. It takes in cool gas into it's suction port and pressurizes it at it's discharge port. The compressor is powered by a drive belt from the engine. The compressor has an electrically operated engagement clutch to either turn the refrigeration operation off or on. Output = high pressure (hot) gas.

Next is the condenser. The condenser is located in front of the radiator (and kind of looks like a radiator too). Through the use of cool air flow provided by the engine fan, the condenser cools the hot gas and converts it to liquid. The liquid is still under considerable pressure and is warm, but not as hot or as high pressure as when it exited the compressor. Output = high pressure (warm) liquid

EXPANSION VALVE SYSTEM - The exiting liquid is sent via a small tube (liquid line) to a receiver/drier (applies only to an expansion valve system). The drier is a can with a desiccant bag inside. It looks about the size of a soda can, and is usually located very near the condenser outlet pipe. There is no pressure/temperature change at the receiver/drier. Output = same as condenser, but moisture is removed by desiccant.

As the high pressure, warm liquid exits the drier, it passes through an expansion device. It can either be an expansion valve (which modulates refrigerant flow in an expansion valve/drier system), or a fixed expansion orifice tube in a cycling clutch/orifice tube system (which expands at the same rate all the time). The two systems have slight have slight functional differences, mainly in how the low temperature is maintained. Well any way, for illustration, the pressurized liquid passes through the expansion device, the pressure is reduced considerably; hence the temperature drops also. Output = Cold Liquid. The following paragraph describes the expansion valve

CYCLING CLUTCH/ORIFICE TUBE SYSTEM - The exiting liquid is sent via a small liquid line DIRECTLY to an expansion ORIFICE TUBE. The orifice tube is fixed, therefore the proportional pressure drop across it will constant. This type of expansion device must work in conjunction with a clutch cycling switch. Because the pressure drop across the orifice tube is constant, the switch is used to maintain the system low pressure side in a certain operating range. The cycling clutch switch, through various pressure changes in the system, turns the compressor off and on during normal operation. A typical operation would have the switch turn the compressor on at about 45 PSI and off at 25 degrees. This would maintain the evaporator refrigerant at temperatures around 35-45 degrees F.

The next step is evaporator operation. As the cold liquid exits the expansion device, it is fed to a heat exchanger type device under the dash that blows warm air from the car interior across it . The cold liquid refrigerant is what cools the air you feel coming out of the ducts. As the air is cooled in the heat exchanger, the liquid refrigerant is heated in the other side of the heat exchanger and then it evaporates. 

ACCUMULATOR - Only used in orifice tube system. It contains a desiccant bag also. The accumulator provides a similar function as the drier in the expansion valve system, but is located in the evaporator outlet instead. This positioning allows the accumulator to collect any un-evaporated refrigerant that may still be in the liquid state, hence protecting the compressor from liquid lock damage.

The evaporated gas then returns via the LARGE tube to the compressor "suction" port to begin this whole process again.

ORIFICE TUBE - The Orifice tube, probably the most commonly used, can be found in most GM and Ford models. It is located in the inlet tube of the evaporator, or in the liquid line, somewhere between the outlet of the condenser and the inlet of the evaporator. This point can be found in a properly functioning system by locating the area between the outlet of the condenser and the inlet of the evaporator that suddenly makes the change from hot to cold. You should then see small dimples placed in the lines that keep the orifice tube from moving. Most of the orifice tubes in use today measure approximately three inches in length and consist of a small brass tube, surrounded by plastic, and covered with a filter screen at each end. It is not uncommon for these tubes to become clogged with small debris. While inexpensive, usually between three to five dollars, the labor to replace one involves recovering the refrigerant, opening the system up, replacing the orifice tube, evacuating, and then recharging. With this in mind, it might make sense to install a larger pre filter in front of the orifice tube to minimize the risk of this problem reoccurring. Some Ford models have a permanently affixed orifice tube in the liquid line. These can be cut out and replace d with a combination filter/orifice assembly.

THERMAL EXPANSION VALVE - Another common refrigerant regulator is the thermal expansion valve, or TXV. Commonly used on import and aftermarket systems. This type of valve can sense both temperature and pressure, and is very efficient at regulating refrigerant flow to the evaporator. Several variations of this valve are commonly found. Another example of a thermal expansion valve is Chrysler's "H Block" type. This type of valve is usually located at the firewall, between the evaporator inlet and outlet tubes and the liquid suction lines. These types of valves, although efficient, have some disadvantages over orifice tube systems. Like orifice tubes, these valves can become clogged with debris, but also have small moving parts that may stick and malfunction due to corrosion.