The Earth's outer structure exists of a crust of solid rock that is split into several plates (tectonic plates), all of which float on a semi molten layer called the mantle.

Convection currents in the Earth's mantle cause movement of these solid plates, which has slowly caused the movement of whole continents over millions of years. Everest is growing by a couple of centimetres every year because of this reason, as the Indian subcontinental plate moves towards the Asian continental plate, the rock is pushed upwards to form the Himalayas.

When these plates move laterally to each other it causes a huge amount of tension to be created between the two rock surfaces. Just like any form of friction, this tension will eventually reach a level of criticality and the force moving the plates will overcome the friction forces. When this occurs, the energy stored in that tension is released very suddenly and causes vibrations throughout the surface of the plate, in the form of an earthquake.

tectonic plates rubbing together gives of vibrations, aren't i clever:)

It happens when to plates deep in the ground hit eachoter and build up pressure until one goes over and the other goes under, then we get a earthquake.ￂﾨ

And a earthquake is when this happens, and then the ground will shake.
The areal depends on how powerfull the quake is...