Geothermal Energy is defined as energy that comes from the water, soil or rock of the Earth. It is a renewable, environmentally friendly and unlimited energy source used to heat and cool homes, commercial / agricultural buildings, or other structures including sidewalks, driveways and bridges.
The three most common geothermal systems are the Horizontal Earth Loop, Vertical Earth Loop, and Pond / Lake Loop. ClimateMaster has a good primer on geothermal energy.