Rooftop or central air conditioning systems rely on a two-part system. The indoor air handler and evaporator uses coolant to absorb heat from the air, and then compresses the coolant before passing it to the outdoor or rooftop unit. There, the coolant releases heat and condenses before returning inside.
Modern air conditioners are essentially one-way heat pumps that manipulate the pressure of coolant to absorb heat efficiently. When the coolant enters the indoor unit, it changes from a liquid into a low pressure gas as it absorbs heat. The compressor condenses this gas, increasing the pressure and the temperature. The condenser coil allows the gas to bleed off heat, returning to a chilled liquid state. In the course of this cycle, heat is removed from the inside of the home, cooling it.
Unlike window air conditioning units, which combine the evaporator and condenser into a single unit, split air conditioning systems are designed to allow some distance between the two halves of the system. In private homes, the condenser is usually situated on the ground just outside the house, but in apartment complexes and business installations, the condenser unit commonly resides on the roof of the building due to the need to situate multiple units in a small area.