Military personnel and DoD civilian employees carry a CAC card for identification (CAC card is kind of like ATM machine in that CAC means Common Access Card). The CAC replaced the old ID card some time ago. There is a microchip embedded on the card and in the Navy and Marine Corps (though I believe the Army and Air Force are going this way if they haven't already) you use your CAC to log into your computer. There is a slot on the keyboard you stick your card into that brings up the log-in prompt.
This is a security measure and works pretty well. As long as you don't forget your card.
You see, since your CAC is also your ID, you have to have it to access the base. But if you are one of the many who forget it and leave it in your computer, then you have some 'spailing to do to the gate guard in the morning.
I've been a government civilian with a CAC for nearly three years now and today I became one of the many who logged out, but forgot to pull out.
Thank God for understanding gate guards.