A heating belt is used to keep the temperature of your brew up to proper temperature when fermenting in cooler temperatures. These belts normally produce about 20 watts and their position on the bucket or carboy determine the amount of heat is put into your brew. The lower the belt is on the fermenting vessel the more heat is absorbed.
Heat belts are more flexible in their use than heat pads as you can control their heat output by their location on the fermenter. Placed lower down they will provide more heat than when placed higher up. (See attached picture for a demonstration of a heat belt in use). Turn the appliance on when the brew is dropping below your desired temperature range and turn it off or move up higher if heat is getting too much.