There are two basic kinds of aquarium heaters. The first is a glass test tube filled with a heating coil and capped with a regulator. The cap attaches to the top edge of the aquarium and the test tube is immersed into the water. Turn the dial and the heating element glows, warming the water. The other kind of heater is a submersible heater that lies on the bottom of the tank. With either one, the trick is to set them so the water temperature reaches and stays where you want it.
For more information, please read the story Keeping Your Tank Warm.