What is the difference between Spring's GA, RC and M2 releases?

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.

SR = Service Release (subsequent maintenance releases that come after major -RELEASE).

Refer to Software release life cycle on Wikipedia. The whole life cycle is described in detail.