Tetonic Plates cover the top layer of the earth. These plates are constantly in motion because of convection currents, causing the plates to shift at around two inches a year. This shifting of the plates result in the formation of earthquakes, mountains and volcanos. Where the two plates connect are called 'boundries'.
There are three types of boundries. A divergent boundry, a convergent boundry and a strike-slip boundry.
A divergent boundry is created when two plates meet but are moving away from each other. A plate that's part of a divergent boundry moves toward another plate, the leading side of that plate becomes a convergent boundry.
At the point of collision of the two converging plates, the more dense plate, usually an ocean plate, gets subducted under a plate supporting land mass. Because of the subduction, the boundry is often called a subduction zone.
The third type of boundry is a strike-slip boundry. At this type of boundry two plates intermittantly rub up against. The resistance and subsequent release of the plates result in an enormous release of energy; and the force of this creates seismic waves the shakes the ground creating an earthquake.