A heat pump is a device that moves heat from one location to another location using a mechanical means. There are two common types of heat pumps: air-source heat pumps and ground-source (geothermal) heat pumps, with variations on both. Both can keep your home warm in the winter and cool in the summer. An air-source heat pump pulls its heat indoors from the outdoor air in the winter and from the indoor air in the summer. A geothermal heat pump extracts heat from the indoor air when it's hot outside, but when it's cold outside, it draws heat into a home from the ground.
A heat pump's refrigeration system consists of a compressor, and two coils made of copper tubing, which are surrounded by aluminum fins to aid heat transfer. The coils look much like the radiator in your car. Like in a refrigerator or air-conditioner, refrigerant flows continuously through pipes, back and forth from the outdoor coils. In the heating mode, liquid refrigerant extracts heat from the outside coils and air, and moves it inside as it evaporates into a gas. The indoor coils transfer heat from the refrigerant as it condenses back into a liquid. A reversing valve, near the compressor, can change the direction of the refrigerant flow for cooling, as well as, for defrosting the outdoor coils in winter.