Until 1995, most cars with air conditioning used R-12 Freon refrigerant which has shown to have an environmental impact. R-12 is no longer used or produced in many countries and remaining quantities have skyrocketed in price. Since then, the industry standard has been R134a, which is an efficient replacement. Contrary to popular belief, most cars originally equipped with R-12 can be converted to 134a and still keep you just as cool.
This is a generic term for the earlier valve regulated systems for the early GM vehicles. In these systems, when the air condition was turned on, the clutch on the compressor was engaged until the air was turned off. When the system reached certain pressures and/or temperatures a valve opened and allows the refrigerant to continue to circulate. This is much different than newer systems where the compressor clutch engages off and on to regulate refrigerant. This is where the old adage of turning off the air to save gas came from. Because no matter what the compressor was working all the time when turned on.
This is a also a generic term for later and modern systems where the clutch on the compressor engages off and on to regulate refigerant. In these systems, whne the air conditioner is turned on, the compressor clutch is engaged until certain pressures and/or temperatures are reached in the system. Once this criteria is met a pressure switch or thermostat opens the clutch power circuit and disengages the clutch, stopping the circulation of refrigerant.