An air conditioner is basically a refrigerator minus the insulated box. Its function - in addition to cleaning and dehumidifying the air entering the vehicle - is to maintain the air at a selected temperature.
So, how does the system keep the car interior just the way we want it? Most people think that an air conditioner simply adds cold air to the interior of the car. Actually, there is no such thing as cold, rather there is an absence of heat. The job of the air conditioner is to remove the heat and humidity that we find uncomfortable.
A compressor circulates a liquid refrigerant, compressing the liquid into a hot, high pressure gas. This hot gas is then run through coils so the heat can be dissipated and condensed into a liquid. The liquid then goes through an expansion valve and evaporates to become a cold, low pressure gas. This cold gas then runs through a set of coils which allow the gas to absorb heat and, in so doing, cool the interior of the vehicle.