Attic fans can keep your home cooler by getting rid of the heat build up in the attic. They rely on two things:
- good attic vents and
- a well-sealed and insulated home.
They use the attic vent to draw cooler air from outside and expel the hot attic air to the outside. Attic air can sometimes be many degrees hotter than the outside ambient air.
If you have a home that is full of gaps between the attic and the rest of the house, and have an air conditioner running, then the attic fan will actually increase your energy use, by drawing the cooler air your air conditioner generates up into the attic.
If you are trying to solve this problem yourself, I recommend the Energy Star website's page (here) on attic ventilation tips for DIY.
For more information:
Read "How do you determine what size fan to put in an attic to cool it properly?" a Q&A answered by Joshua Lloyd.