The Earth gives us heat in many important ways. The two we use most commonly now are the constant 55-degree temperatures it maintains near the surface, and the intensely hot magma it sustains at its core.
We tap the constancy of the Earth's crust with simple tubing, and even by digging our homes and buildings into the ground. In very cold climates it has long been common to build houses partly or wholly below the Earth's surface. Some are also partially covered by dirt berms that protect against the extreme weather.
By maintaining a constant moderate temperature level, these designs save substantial amounts of energy that would otherwise be required for heating. Supplemental power can be used to raise temperatures above 55 degrees, but naturally maintaining a steady heat makes a big difference. When the weather gets hot, the tables are turned, and the 55- degree level makes for easier cooling.
The Earth's moderating energies can be otherwise tapped by digging down 50 feet or more and using metal or other tubing to run water or air into the ground, and then back up. Heat exchange mechanisms can then bring steady temperatures into buildings of all varieties.
In a sense, new "green roofs" are also a form of geothermal energy. By covering rooftops---however high in the air---with sod or other Earthen surfaces, the sun's intense heat and the winter's bitter cold can be moderated. Even large industrial facilities, such as a Ford factory in Michigan, are finding Earthen rooftops to be extremely beneficial.
A more dramatic form of geothermal energy comes from digging very deep toward the Earth's core to tap into regions heated by the power of the core magma. We're familiar with this power from the astonishing force of volcanic eruptions and the super-hot lava that comes with them. Tapping into that power can be complex and demanding. But capturing just a tiny fraction of it can be hugely profitable.
Geothermal energy in this form has long been used in Italy, Iceland, California and elsewhere in a wide range of industrial, structural heating and other applications. When used in producing electricity it can emit 3% or less per kilowatt-hour of the carbon dioxide emissions as produced by a fossil fuel plant.
Like everything else, there are ways to make a mess of it. A geothermal well proposed for the Big Island of Hawaii drew bitter opposition from the native community, which considers the spot to be sacred. In some instances, environmental issues such as noxious odors and potential impacts on nearby waters have also been important.
But overall, the moderating power of the Earth's crust and the intense heat of its core are almost certain to become increasingly important sources of clean, green energy---if we do them right.
Resources:
Department of Energy-Geothermal
http://www1.eere.energy.gov/geothermal/
Geothermal Energy Association
http://www.geo-energy.org/
Geothermal Education Office
http://geothermal.marin.org/pwrheat.htm