Why do Ice Ages occur?

This question is really two separate questions:

1) Why were there four long, generally cool periods during which continent-sized glaciers advanced and retreated?
Learn about plate techtonics, continental uplift, reduction of CO2, and changes in the earth's orbit.
2) What controls the advance and retreat of large ice sheets during the four long, cool periods?
Learn about the eccentricity of the earth's orbit, the tilt of the earth's axis, and the precession of the equinoxes.

In combination, all these shifting and cycling factors influence the cooling and warming of the earth's crust.