The Plates of Lithosphere
Tectonic plates are make up earth's crust, aswell as the upper part of the mantle. The crust and upper mantle are called the lithosphere. They are about 80 km deep. The lithosphere is broken into giant plates that fit around earth, in other words there are pieces filling up earth. These pieces move a tiny bit each year as they slide on top of a liquidy part of the mantle called the asthenosphere. When all that rock moves it can cause an earthquake.
http://www.emsb.qc.ca/rosemount/steinberger/lithosphere.html