I will answer from the Navy-Marine Corp side, only to tell you that the web site mentioned previously explains quite well what the mission of MARS is.
In short, it is Emergency Communications.
MARS operators have, in my local area, practiced in a HAZMAT drill, a terrorist attack, and a radiological dispersion device drill. In the latter, they were the link between the EOC and the military unit assisting our Incident Commander.
I hope you can see the value of MARS and will join, regardless of the service you choose. While you indicate a desire to join Army MARS and have the web site to do so, I will, for subsequent readers, point you to
http://www.navymars.org where you will find links to both Air Force and Army MARS.
Let me know how more I can help.
Lloyd Colston Mayes County CEM
Pryor, OK USA
http://www.geocities.com/mccem Homeland Security begins at Home.