The Earth is made up of many plates which move over the Earth's surface.
The plates move because of convection currents which circulate in the mantle. There are several sources of the heat such as heat left over from the formation of the Earth and heat produced because the rotation of the Earth is slowing down so kinetic energy is being turned into heat energy, but the main source of heat is from the decay of radioactive elements. The convection currents drag the overlying rigid lithosphere along.
They also move due to gravity. The heavier rocks sink and are subducted while the lighter, younger, hot rock rises. The rising magma forces the plates apart resulting in the formation of a constructive plate margin, e.g. Mid-Atlantic Ridge.
Constructive Plate Margins
Destructive Plate Margins