Geological plates are enormous slabs of solid rock that make up the earth's crust and outermost layer, the lithosphere. They are more commonly referred to as tectonic plates.
Tectonic plates are composed of continental and oceanic lithosphere. The sizes of the plates can vary greatly and change over geological eras. The plates' movement over the mantle is fed by heat and pressure from the Earth's core. Tectonic plates either collide or move apart from each other. When they collide, the lighter and cooler plate slowly moves underneath the other plate and eventually disappears completely. This is how mountains form and why areas around the boundaries of tectonic plates experience earthquakes and volcanic activity.