Termites are the only insects that actually eat wood. The Termite's function in nature is to remove dead trees and limbs to make way for new trees. In their search for wood, termites sometimes enter your house and use any cellulose material such as wood or the backing on your sheetrock for their food source.
The Termites in our area are subterranean species. They live underground. In order for termites to enter your house, they usually build a tube constructed from the soil around your house and they use this tube to travel back and forth between the ground and your home. In most cases, unless you have a moisture problem in your home, termites must have this contact with the ground to obtain the moisture they need to survive.