A geothermal heat pump is a heating and cooling machine. Geothermal heat pumps heat and cool your home by moving heat between the earth and your home. A geothermal heat pump does the same thing as an air-source heat pump, except it uses the earth instead of the air for heat transfer (geo means earth, and thermal means heat). The heat that it transfers into the home in the winter is heat that the earth has absorbed from the summer sunshine. Since you already own this heat, it's free for the taking - you only pay to concentrate it and move it inside the house. A geothermal heat pump is the only way to gather up and use this free source of heat.
Geothermal heat pumps can be the most energy efficient (and environmentally clean) way to heat and cool your home, but they must be matched closely to your home, and the earth it is coupled to, to achieve this. Because every homesite is unique, there are no rules of thumb for the design of a geothermal heat pump system.