Split system units are the most common form of air conditioning. This is because they're easy to install, simple to use and very efficient. Split systems are comprised of two parts that are split between an external unit and an internal unit. The external unit pumps air from outside in through its compressor, turning it from warm to cold or cold to warm. This air is then dispersed throughout your room from the head unit that is mounted on your wall. They are effective for heating or cooling open living spaces and single rooms. The temperature can be easily set by the press of a button.