Your air conditioning system is made up of a compressor, a condenser, an evaporator (or drier), refrigeration lines, and a couple of sensors here and there. Here's what they do:

• Compressor: This is the heart of your AC system. The compressor is what takes the refrigerant (the gas) and pressurizes it, so it will absorb heat and cool the air. It is run by an engine belt. The compressor also has an electrically-operated clutch that turns the compressor on and off as you demand more or less cool air.
• Condenser: The condenser is like a miniature radiator, usually mounted at the front of the car right next to your big radiator. Sometimes the condenser will have its own electric cooling fan, too. The hot, compressed air passes through the condenser and gets much cooler as it dissipates the heat it's carrying. As it cools, the gas condenses back into a liquid.
• Evaporator: The evaporator is another little radiator that serves exactly the opposite task as the condenser. As the super-cool liquid is passed through the evaporator's tubes, air is forced through and gets really cold, right before it blows into the cabin of your car. As the liquid refrigerant warms up again, it starts turning back into a gas and continues to circulate through the system.
• Thermal expansion valve: To control the air temperature, the AC system has a valve that controls the flow of super-cool refrigerant to the evaporator. This allows your car to regulate just how cold the blowing air gets. There are several types of valves, but they all do the same thing.
• Dryer or accumulator: The dryer, also known as the receiver-dryer, serves as something like a safety catch for your system. Although the compressor is intended to compress only the gas form of your refrigerant, there's always a chance that some liquid could make it back to the dryer. The dryer catches this liquid before it can damage your compressor. Since even the tiniest leak or careless installation can introduce water moisture into the system, the dryer absorbs this chemically using what's called a desiccant. The dryer also has a filter that catches any contaminants that might be in the system. 

All air conditioning systems have these basic parts, although different systems use various forms of sensors to monitor pressure and temperatures. These variations are specific to the make and model of different vehicles. If you need to do some work on your car or truck's AC system, be sure to have a repair manual specific to your vehicle.