Both "Ground Source Energy" and "Geothermal Energy" are terms used to describe energy which is sourced from the Earth, so it's not surprising there is some confusion. However, true geothermal energy is tapped from the Earth's central molten core. In Iceland whole townships are heated using the geothermal heat from subterranean volcanic systems. The term " Ground Source Energy" is not used to describe heating sourced from the Earth's molten core.
In addition to the heat emitted from the Earth's central core, the Earth acts as a huge solar energy collector absorbing 47% of the sun's energy. This is 500 times more energy than mankind uses every year and all in the form of clean renewable energy.
Only a few metres down the temperature tends to remain constant at about 10-16°C and usually varies by a few degrees (more in the winter) from the surface and air temperature. This is the principal on which a Ground Source Heat Pump works. The amount of temperature variance depends on the geology of the soil, climate and seasonality.
By using the Earth's vast energy store it is possible to greatly reduce a building's energy consumption for heating and cooling services and in turn reduce associated CO² emissions.