Air conditioning is a component of the HVAC (heating, ventilating and air conditioning) system. The air-conditioning unit sits outside of the home and pumps liquid refrigerant (a liquid that cools something -- often referred to as "Freon") into the air-handling unit (AHU), the unit inside the home that generates heated or cooled air. The AHU generates cool air and forces it through the home via supply ducts and into rooms. (Ducts are passageways, usually tubular and made from sheet metal, flexible material or rigid insulation, that deliver air from the AHU throughout the home, and also return air from the rooms back into the AHU for re-circulation.) Meanwhile, the slightly warmed refrigerant travels back outside to the air conditioner, where it's cooled and re-circulated.
It's important to note, however, that even a high SEER unit may not perform well if the overall HVAC system, including how the air is distributed through the home, hasn't been well planned. As part of the HVAC system, duct work plays a critical role in delivering cool air to the rooms. A state-of-the-art air conditioner is only effective if the air it cools reaches the family.