How'd you guess??
There's a whole lot of reserves that no one on AD knows about.
IRR = Inactive Ready Reserve. This one is the one that most guys on AD know about. You get out, accept a reserve commission, and you're in the IRR. You get out prior to 8 years of total service, you're in the IRR. You don't do a thing, unless you want to.
SMCR = Selected Marine Corps Reserve. This is the one that most AD guys identify as the reserves. Drilling reserve units that people affiliate with. There's Infantry Battalions, Engineers, Squadrons, etc...
IMA = Individual Mobilization Augmentation. This is the one that almost no AD guys know about. You name it, there's an IMA billet doing it - helping out the active component. Our squadron XO just took a 1 year IMA billet with MCLL in Afghanistan. I know two people flying C-12's on IMA billets. VMR-1 has IMA billets for Phrog pilots flying the HH-46E. HMX-1 has IMA billets. They're everywhere, CONUS and OCONUS.