An attic fan circulates the air. It's especially good at pulling the hottest air (that's risen to the attic) out of the building. This also draws the cooler air up.
A fan uses less power than air conditioning. So for weather that is only a bit too warm, the attic fan can provide enough cooling. When the weather is hotter, or more cooling is desired, the attic fan can make the air conditioning more efficient.
- wikiHow on How to Use a Whole House Fan