The Earth
The Earth is about 13000km across, and has a very interesting structure. In the centre is a hot molten core, which is extremely dense. Moving outward we pass through the mantle, a solid yet ductile material. Then, at the top, we have a very thin crust.
The whole thing moves around and, by friction (and other physical effects like radioactive decay and current loops, but don't worry about those), heat is actually generated.
At the surface, we see the effects of the motion within the Earth. Mountains are formed as two tectonic plates move together, and earthquakes occur along unstable fault lines. As a move obvious example of the power contained within the earth, just look at geysers and thermal vents.
Geothermal Power Plants
The idea is to drill down into the earth far enough so that water you pump down there comes back up very hot. The steam that comes off this water drives a turbine, cools, condenses, and goes down again to repeat the process.
This is a long term goal, and is technological difficult. There are other ways of harnessing energy from the earth, such as ground heat pumps.
Since the ground has such a large thermal inertia (i.e. it doesn't gain or lose energy quickly), it retains a fairly constant temperature all year around, which can conveniently be used to cool houses in summer and heat them in winter.
Return to alternative energy sources from what is geothermal energy