A severance package is basically a settlement from the company, it is a negotiated sum of money and/or benefits for a period of time in exchange for an agreement that the employee will not attempt to collect unemployment and/or sue for wrongful termination.
They are a rare gift, I would be happily surprised if offered one, less the being terminated part of course.
From those who I have known that have received severance packages, the package is usually a sum greater than what they would have received from unemployment. My father in-law used to work for MCI and received a total of 5 severance packages, being terminated and then rehired again and again. He would joke that they were like extended paid vacations.