What is an earthquake?

An earthquake is the sudden release of energy stored in rocks that results in the movement of those rocks along a fracture in the rocks called a fault. In most cases, the energy needed to move those rocks accumulated very slowly over decades, centuries, or even longer as a result of plate tectonic processes. During an earthquake, much, if not all, of that slowly-accumulated energy is released suddenly, in the matter of seconds to minutes.  

The above definition differs from some older ones you might find in classic seismology books. Early on in the science of earthquakes, when the precise causes of earthquakes were unknown, earthquake often referred to the shaking - today we refer to the "source" of the shaking as the earthquake. The vibrations we feel are the result of seismic waves propagating outward and transmitting energy from the region where the rocks slid along the fault. 

Earthquakes come in all sizes, ranging from those that produce microscopic motions on tiny fractures to large rock movements of 10's of meters extending over very large areas. They are part of a broad spectrum of deformation processes, many of which occur much more slowly, with less devastation. The map below shows the location of moderate and large shallow (<70 km depth) earthquakes. 

Most people are aware of the association of earthquakes with plate boundaries. About 90% of earthquake activity is associated with plate-boundary processes, and most of the other 10% is associated with former or future plate boundaries. The map below shows the modern configuration of Earth's major tectonic plates.

The ultimate source of the energy for earthquakes is the cooling of Earth, which drives plate tectonic processes.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.