The main functions of a ceiling fan are to evenly circulate the air in a room and create a breeze. As air is moved throughout a room, it causes a more efficient release of body heat and makes the air inside a home seem cooler without affecting the actual temperature.
Varying positions of air conditioning intake vents and supply registers, coupled with the fact that hot air rises and cool air falls, causes uneven temperature distribution of interior air. Using a ceiling fan to circulate the air will help achieve a more consistent mixture of air to be conditioned, resulting in a more efficiently operating AC.
Ceiling fans are manufactured with a switch on the side of the housing that will reverse the direction of fan blade rotation. During the cooler months, when you are using a heater, running the fan on low speed in reverse will help force the warmer air at the ceiling down to the living space of a room.