AHUs or air handling units are the components used by an HVAC system to distribute hot or cold air via ducts. On the air hand, FCUs or fan coil units are standalone components that distribute air directly without any ductwork.
These are typically mounted on the ceiling or wall of your home, and they have a filter, refrigerant coil, and blower fan of their own. Furthermore, an FCU is connected to the outdoor unit of the AC via a refrigerant line.
As you may have guessed, you will find FCUs in a ductless AC, but there are many units that come without them.