Well since i have a great book about earthquakes : Soviet Great Encyclopedy book 16. I will write here my own simplified version, but you can also read wikipedia.
To understand something fully, we need to answer following questions: What is it? What is it caused by? How does it affect its enviroment? How can we measure it?
Earthquake is basically a series of "pushes" that are coming from underground or "wobbling" of land. We can measure strength of those pushes by Richter magnitude scale, well if we dig even deeper it measures aplitude of lands shaking. Intensity of lands shaking is measured by Mercalli scale. It is also possible to measure earthquake strength by the damage that it causes.
So next question that we have to answer is after which event does earthquake appear? Reason for those pushes from underground are tectonical movements. It can be explained by relation of earthquake strength and "folded" areas, more "folded" area is the stronger are earthquakes there.
There are few words that are useful to know when we are speaking about earthquakes:
Hypocenter - is the area, which is located underground, where the earthquake bore.
Epicenter - is area on the land, where earthquake "pushes" are strongest. It is located right above hypocenter.
Basically earthquakes only bring destruction, depending on the strength of earthquake there sometimes reaaaally dramatic endings. For example:
In China in 1556 several hundred thousands of people died.
A lot of damage have suffered Lisabon in 1755, San-Franciscko in 1906, Messin in 1908, Tokyo in 1923.
After strong earthquakes there can be strong hum, changes in electric and magnetic field of earth, also sometimes there are flashes.
Strength of earthquake and area that it is influencing are not related to each other. It all depends on how deep hypocenter is located. Earthquakes also appear sometimes when there is volcanic activity. They are caused by collapse of karst cavities. Normally those earthquakes are not very strong.
Soviet sciencists have found out how earthquake really works. Earthquake is the result of slow and continious accumulation of tectonic stress, which when achieve the ultimate strength of rocks, that combine given land area, lead to ruptures in land and sudden shifts of masses along the weak areas. Basically earthquake is process of discharging of the accumulated stress.
So that is it, here come some pictures...