From what I remember they check all your past employers, your credit history, driving record, criminal record, and a bunch of other stuff.
It used to be that they did your check before they hired you. I had a buddy of mine get delayed on the hiring because he was waiting on his DMV stuff. They had enetered his name wrong in the computer and it kept coming up as nobody listed.
He had to work it out himself, just make sure you keep on top of it and be available if they have any problems.