Heat pumps are devices that use relatively little energy to transfer heat from one place to another. They work best in moderate climates, where they can be used in place of a furnace or air conditioner in order to save on your utility bills. Some heat pumps don't work well in cold climates, so it's important to investigate what type of heat pump works best in your climate. With the wrong type of heat pump, you might end up spending more on energy than you did before you installed it.
Heat pumps work by pulling heat out of the ground or the air in order to warm a house or office building; in the summer, they can be reversed in order to cool the same space. The reason heat pumps are considered so efficient is because they merely transfer heat; they don't have to burn any fuel in order to create it.
The reason that heat pumps aren't very effective in climates where the air temperature dips close to freezing on a regular basis is because it takes a lot more energy to move heat from a very cold area to a hotter one. It's much easier to move heat between places with a minimal temperature difference. Plus, in moderate climates there's more heat outside to bring in. When it's cold out, it's harder to extract the heat from the air. If the heat pump can't get enough heat from the outside air to warm your house, you have to use supplemental energy in order to get your house to a comfortable temperature. This supplemental heating can be electrical, or it can burn oil or gas. The type of heating used most in your area is probably your best bet for a backup.