Unlike air conditioners, fans only move air -- they do not directly change its temperature. In summer, the fan's direction of rotation should be set so that air is blown downward. The breeze created by a ceiling fan speeds the evaporation of perspiration on human skin, which makes the body's natural cooling mechanism much more efficient. Since the fan works directly on the body, rather than by changing the temperature of the air, during the summer it is more energy efficient to only use the fan when you’re in the room. Turn it off when you leave.
In winter, ceiling fans should be set to turn the opposite direction and used on a low speed. Air naturally stratifies — that is, warmer air rises to the ceiling while cooler air sinks. A ceiling fan, with its direction of rotation set so that air is drawn upward, pulls up the colder air below, forcing the warmer air nearer the ceiling to move down to take its place, without blowing a stream of air directly at the occupants of the room. This action works to even out the temperature in the room, making it cooler nearer the ceiling, but warmer nearer the floor. Thus the thermostat in the area can be set a few degrees lower to save energy, while maintaining the same level of comfort. It is important to run the fan at a low speed to minimize the wind chill effect described above.