In a listing of earthquakes, what do the different letters after the magnitude mean?
There are different ways of measuring magnitude and each way is notated with its own abbreviation:
Mw - estimates of the seismic moment
Mb - maximum amplitudes of teleseismic body waves
Ms - surface waves
Ml - local recordings
Md - signal duration
The USGS website has a FAQs section that answers some common questions about earthquakes and how they are measured.