what happens if your financed car gets totaled and you still owe money?

Been thinking about this lately... say you finance a car, you're making payments, everything's fine, then bam—someone rear-ends you and the car's totaled. Insurance pays out, but it's less than what you owe on the loan. Um, does anyone know how that works exactly? Like, do you just keep paying for a car you don't even have anymore or is there some kind of insurance that covers the difference? Seems kinda scary to think about, you know?