From Open Energy Information
Geothermal energy is heat extracted from the Earth [Geo (Earth) + thermal (heat)].The temperature of the Earth varies widely, and a wide range of temperatures can be suitable for using geothermal energy, from room temperature to above 300° F. This heat can be drawn from several sources, ranging from the shallow ground (the upper 10 feet beneath the surface of the Earth) that maintains a relatively constant temperature of approximately 50° to 60° F, to reservoirs of extremely hot water and steam located both near the Earth's surface as well as several miles deep into the Earth, even reaching the Earth's magma. Geothermal reservoirs are generally classified as either low temperature (<302°F) or high temperature (>302°F). Commercial electricity production normally requires a high-temperature reservoir capable of providing hydrothermal (hot water and steam) resources, called hydrothermal reservoirs. Geothermal is distinct from other renewables such as solar or wind because it can provide electricity 24 hours a day, 365 days a year.
Geothermal Energy Basics
The following information was taken from NREL's Geothermal Energy Basics page.
Many technologies have been developed to take advantage of geothermal energy. This heat can be drawn from several sources: hot water or steam reservoirs deep in the earth that are accessed by drilling; geothermal reservoirs located near the earth's surface, mostly located in western states, Alaska, and Hawaii; and the shallow ground near the Earth's surface that maintains a relatively constant temperature of 50°-60° F.
This variety of geothermal resources allows them to be used on both large and small scales. A utility can use superheated hot water and steam from deep geothermal reservoirs to drive generators and produce electricity for its customers. Other applications apply the heat produced from geologic geothermal features directly to uses like snow melting on roads, agricultural needs, and industrial processes. Finally, geothermal heat pumps (GHPs) tap the heat energy stored everywhere in the ground. In summer, they use the ground as a heat sink and in the winter as a heat source for homes and other buildings. GHPs cut energy cost in buildings by 40 to 70 percent. With such savings, GHPs provide an economical, renewable and reliable alternative to standard heating and cooling systems.
Other geothermal resources exist miles beneath the earth's surface in the super high-temperature rock and magma that is found there. With new technoogies in the future, these resources may also be useful as sources of heat and energy.