From Snell's website:

Snell SA2000 - M2000: Our customerss have asked what the difference is between a Snell SA2000 and M2000 certification for a helmet. All Snell tests for these specifications include multiple impact tests at -20 degrees, 78 degrees (dry and wet) and 120 degrees. The Snell M2000 specification is the base for the specs. The SA2000 - used primarily for auto racing - adds not only the thicker shield, but also the use of flame retardant paint, trim, retention strap and interior. The Snell SA2000 helmet also meets an additional rollbar impact test.


A link describing both tests...
Snell and DOT Certification Ratings - Everything You Want To Know About ATV Helmets
No info on what the ratings mean, but it explains the differences between DOT and Snell.

Here's another one. I'll take the Snell rated one, okaythanks.
DOT helmet, Snell rated motorcycle helmets