Lady's Human is correct, in that in the past the people who were not working did receive backpay for the time they were off. But this time there is a real possibility that people not working may not receive their backpay. To authorize backpay, both the Senate and House have to agree to it. Given the current state of affairs, there is a feeling that this may not happen this time. People who are deemed "essential" and are working during the shut down (such as the military, air traffic controllers, and meat inspectors), will not receive a pay check while the government is closed. But they will receive their backpay when the government re-opens. So they are the "fortunate" ones as long as the shut down does not last too long, they should be okay.

BTW, all the legislators are deemed "essential" so they will get their pay...(but their janitors are not)

For the sake of all the people who will be laid off as of noon today, I hope that this is resolved quickly in a sensible manner and we do not have the same problem on October 19th (when the debt limit is reached).