Earthquakes are the sudden violent shaking of the ground. This happens because the Earth's plates are constantly moving and sometimes, because of friction, plates become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released it sends out huge amounts of energy, causing the Earth's surface to shake violently.
The point inside the Earth's crust where the earthquake originates from is known as the focus. The earthquake's energy is released in seismic waves and spread out from the focus. The seismic waves are most powerful at the epicentre. The epicentre is the point on the Earth's surface directly above the focus.