What is magnituda?

Magnitude earthquake - the notional value that characterizes the energy that is released during an earthquake as seismic waves.  Magnitude calculated on records of oscillations recorded by special devices - seismographs.  First magnitude scale was proposed by an American seismologist Charles Richter in 1935, so the value is often called the magnitude on the Richter scale.  Magnitude no units, so the expression "5 on the Richter scale" is incorrect.  In the logarithmic scale used scale, so magnitude increase in value per unit of the increase in the amplitude 10 times and an increase in dedicated energy during an earthquake 32 times.  

Example record earthquakes from Romania, magnitude 2.5.

Example record earthquakes from Romania, magnitude 5.5.

It should be noted that magnitude does not fully characterize the effects of the earthquake on the earth's surface, depending on many factors, first and foremost, the depth of the source.