Earthquake Magnitude Descriptions

Magnitudes can be based on any of the following:

Mw - estimates of the seismic moment
Mb - maximum amplitudes of teleseismic body waves
Ms - surface waves
Ml - local recordings
Md - signal duration

More Information
The USGS website has a FAQs section that answers some common questions about earthquakes and how they are measured: http://earthquake.usgs.gov/learning/faq.php