Air conditioning (AC) is used to cool air in offices and homes. A compressor compresses a refrigerant gas such as freon so that it heats up.
This then runs through an expansion valve, cooling it down. The colder gas then passes through coils and absorbs the heat from air that has been extracted from the building, cooling it down. This cool air is then blown back into the building. (see related terms below - central air conditioning, air conditioner, condenser or outdoor coil, evaporator or indoor coil)