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I don't usually use the term "geothermal" for this, since I think of "geothermal" as getting energy from a volcanic-related heat source (very hot.) But it does seem to be used in this context. This site uses the term "geoexchange."
What it does do is reach into the ground to find that level where the temperature is stable all year around (in the midwest, warmer in winter and cooler in summer). Pipes circulating in this zone allow the use of heat pump technology where it would normally not be effective.
The hardest part would seem to be laying the pipes.
In moderate climates, this system is less useful, since the outside air is usually temperate enough for normal heat pumps to work.