Stop Loss from usmilitary.about.com

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6. Once you complete your enlistment you can get out and won't be called back again.

Truth: Everybody (and I mean EVERYBODY) who enters the military for their first time incurs a total eight-year service commitment. It doesn't matter if your contract says you're enlisting for two, three, four or five years active duty, you are obligated for a total of eight years. If you sign a six-year Guard/Reserve contract, and elect not to reenlist at the end of the six years, you will still be obligated for an additional two years.
Time not spent on active duty, or in the drilling reserves is spent in the IRR (Individual Ready Reserves). While in the IRR, one does not get paid, nor do they perform drill, but can be involuntarily recalled to active duty at any time. Right now, only the Army and Marine Corps have been recalling IRR members. The Army has recalled about 6,000 IRR members and the Marine Corps about 1,000. The Air Force, Navy and Coast Guard is not currently involuntarily recalling IRR members.
In addition to IRR recalls, a program called STOP-LOSS allows the service to prevent (delay) separations and retirements during times of conflict. The Army and Marine Corps place individuals under STOP-LOSS when the person/unit is officially notified of an upcoming deployment (usually about 90 days before the deployment date) until 90 days following return from the deployment. The Air Force, Navy and Coast Guard do not currently have any STOP-LOSS programs in place, but have used it in the past.