Geothermal technologies use the clean, sustainable heat from the Earth. Geothermal resources include the heat retained in shallow ground, hot water and rock found a few miles beneath the Earth's surface, and extremely high-temperature molten rock located deep in the Earth.
For a technology overview try:
DOE Geothermal Technologies Website:
Geothermal Energy Association - http://geo-energy.org/Basics.aspx
For geothermal industry news try:
Think Geothermal - http://thinkgeoenergy.com/news