Geothermal energy, the heat inside the earth, is captured via power plants that generate steam to make electricity, or via heat pumps.
In the former, utility-scale method, wells are drilled deep into the earth to pump steam or hot water to the surface at high pressure. At the surface, the pressure is dropped, causing the water to turn into steam. The steam spins a turbine connected to a generator, producing electricity. In a cooling tower, the steam condenses back into water, which is then pumped back into the earth to begin the process anew.
Geothermal power plants are usually found in areas with hot springs, geysers, or volcanic activity. In the United States, they are primarily situated in the western states and Hawaii, with California generating the most electricity from this renewable source.
At the residential level, ground source heat pumps can heat and cool houses, and heat swimming pools. In this setting, heat is transferred by pumping water or a refrigerant through a loop of vertical or horizontal pipes buried just below ground level, where the temperature is constant. During the winter, the liquid absorbs warmth from the earth and it is pumped to the structure above. The liquid cools following the heat transfer and is pumped back underground, repeating the process. In warm weather, the process runs in reverse.
Ground-source heat pumps run on electricity, a more environmentally friendly heating and cooling system than fossil fuel-based technology, depending upon how that electricity is powered. If powered by renewables, the electricity is emissions-free. Geothermal energy works in most climates, and can offer up to a 70% reduction in heating and a 50% decrease in cooling costs. Up-front installation costs are high, but systems generally pay for themselves in five to seven years. Ground-source heat pumps can require alterations to the landscape, and open-loop geothermal systems, while uncommon, can contaminate groundwater.
For climates with moderate heating and cooling needs, you can install heat pumps without geothermal (and, therefore, without digging into the ground). Air-source heat pumps move heat, in the winter, from the cool outdoors into your home and, in the heat of the summer, move heat from your home into the outdoors. A vastly cheaper way to heat and cool. Much more here.
A Canadian company recently sent us a heat pump glossary which you might find useful.
State and municipal level goals, and mandates to transition from fossil fuels to renewable energy, are among the factors expected to spur growth in geothermal energy.