The angle of the fan blades and the spinning direction determine whether it pushes (blows) air downward or upward.
Some fans are designed to reverse direction, and some are designed to change the blade angle to switch air flow from downward (for cooling) to upward (for heating season air movement).
Moving air can make the air temperature feel cooler, reducing the need for more air conditioning. In the winter reversing the direction of the blades and having the air blow up will allow the air in a home to be circulated which will reduce cold spots but also reduce the cool drafty feel when air blows directly past you.
Check out Choosing and Using Ceiling Fans to Reduce Energy Use for more information.
