Taking it all the way back to Greek roots, Wikipedia says the adjective geothermal originates from the Greek roots (ge), meaning earth, and (thermos), meaning hot.”
Technologically speaking, geothermal is a means to use the natural heat from the earth to generate electricity, air conditioning, heat, or all of the above. The earth is a great source of heat and a great heat sink, which means it can absorb astronomical amounts of heat energy. Although the earth seems like rock, there are many crevices and gaps in this rock. Deep in the earths crust (many hundreds of feet) there is superheated water. This water is at a very high temperature and pressure.
Geothermal brings this water to the earth’s surface and lowers the pressure in a controlled environment. The lowering of pressure creates steam, which can turn huge turbines, creating electricity. After this steam and the other water cools, it is pumped deep into the earth to be reheated. This is how we use geothermal to create electricity.
Geothermal can also be used to heat and cool our houses, schools, hospitals, and buildings of all shapes and sizes. Traditional HVAC equipment produces heating and cooling by transferring heat to and from our atmosphere. This is ok, but the atmospheric temperature can vary a lot. Trying to air condition when it is hot out is difficult. So is heating when it is cold out. Geothermal uses the ground temperature instead of the outside air temperature. The temperature a few hundred feet in the earth only varies a few degrees. Geothermal heats and cools using this constant temperature as its source.
The main thing you need to know is: Geothermal means “Heat from the earth.” There are many ways we can use it.