Magnitude indicates the severity of an earthquake using the Richter Scale, a logarithmic, instrumentally determined measurement. Magnitude rates an earthquake as a whole. The severity of an earthquake is a rating based on the amplitude of the seismic waves. Larger amplitude waves equals higher magnitude earthquake equals greater severity. The Mercalli Scale defines intensity.
Intensity is rated by how much damage was caused by an earthquake and how it affected people.
Go to the University of Nevada-Reno Richter scale page for an excellent explanation of the Richter Scale and other earthquake quantifying tools. The UNR Seismological Laboratory Page is full of interesting information for earthquake enthusiasists. Look up Recent Earthquakes on the internet. Click the picture for more images of earthquake damage. Primary effects are permanent features produced by the earthquake.