Refer to Software Release Life Cycle
GA = General availability (a release); should be very stable and feature complete
RC = Release candidate; probably feature complete and should be pretty stable - problems should be relatively rare and minor, but worth reporting to try to get them fixed for release.
M = Milestone build - probably not feature complete; should be vaguely stable (i.e. it's more than just a nightly snapshot) but may still have problems.
