The Earth has four main layers - the inner core, the outer core, the mantle and the crust.
It was once believed that heat from the Earth's core caused convection currents in the mantle and that these currents slowly moved the crust around.
It is now thought that plate movement is driven by a mechanism called slab pull. Slab pull occurs where older, denser tectonic plates sink into the mantle. As these older sections of plates sink, newer and less dense sections of plate are pulled along behind. Sinking in one place leads to plates spreading apart in other places.
Earthquakes and volcanoes are primarily found at plate boundaries. The plates are like giant rafts that slowly move around. At the boundaries between plates, molten magma is able to force its way to the surface and escape as lava.