A geothermal heat pump is a type of heat pump that uses the ground, ground water, or ponds as a heat source and heat sink, rather than outside air. Ground or water temperatures are more constant and are warmer in winter and cooler in summer than air temperatures. Geothermal heat pumps operate more efficiently than conventional or air-source heat pumps. 
Geothermal heat pumps (sometimes referred to as GeoExchange, earth coupled, ground-source, or water-source heat pumps) ahve been in use since the late 1940's. Geothermal heat pumps (GPH's) use the constant temperature of the earth as the exchange mediuminstead of the outside air temerature. This allows the system to reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days. 
While many regions experience seasonal temperature extremes, from scorching heat in the summer to sub-zero cold in the winter - a few feet below the Earth's surface the ground remains at a relatively constant temperature. In the U.S. depending on latitude, ground temperatures reange from 45 degrees F to 75 degrees F. Like a cave, this ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. The GHP rakes advantage of this by exchanging heat with the earth through a ground heat exchanger. 
A dual-source heat pump combines an air-source heat pump. These appliances combine the best of both systems. Dual-source heat pumps have highter efficiency ratings than air-source units, but are not as efficient as geothermal unites. The main advantage of dual-source systems is that they cost much less to install than a single geothermal unit, and work almost as well.
Benefits of geothermal heat pump systems 
The greatest benefit of GHP's is that they use 25%-50% less electricity than conventional heating or cooling systems. This translates into a GHP using one unit of electricity to move three units of heat from the earth. According to the EPA. geothermal heat pumps can reduce energy consumption - and corresponding emissions - up to 44% compared to air-source heat pumps and up to 72% compared to electricresistance heating with standard air-conditioning equipment. GHPs also improve humidity control by maintaining about 50% relative indoor humidity, making GHPs very effective in humid areas.
Geothermal heat pump systems allow for design flexibility and can be installed in both new and retrofit situation. Because the hardware requires less space than that needed by conventional HVAC systems, the equipment rooms can be greatly scaled down in size freeing space for productive use. GHP systems also provide excellent "zone" space conditioning, allowing different parts of your home to be heated or cooled to different temperatures. 
Because GHP systems have relatively few moving parts and because those parts are sheltered inside a building, they are durable and highly reliable. The underground piping often last 20 years or more. Since they ususally have no outdoor complressorsm GHPs are not susceptible to vandalism. On the other hand, the components in the living space are easily accessible, which increases the convenience factor and helps ensure that the upkeep is done on a timely basis. 
Because they have no oitside condensing units like air conditioners, there's no concern about noise outside the home. A two-speed GHP systemis so quiet inside a house that users do not know it is operation: there are no tell-tale blasts of cold or hot air.