In the old days, almost any US airline pilot (and some 135 guys too) could show up at the gate for any US airline flight, show their badge, fill out a form and get a free ride...if there is a seat available in the cabin, you usually sit there (maybe first class even). If the cabin is full you could ride in the spare seat in the cockpit (some planes actually have two of these).

Post 9/11, you can still do this, but in many cases you may only occupy a seat in the cabin...if it's full in the back you're out of luck. In some cases you can still ride in the cockpit jumpseat if certain security criterea are met.

The background check is the one that all airline pilots have when they get hired. You must be an airline (121) pilot to jumpseat. A few government and airline employees can also jumpseat on a limited basis. Some companies (including mine) will allow 135 pilots to "jumpseat", but because those guys don't necesarily have any kind of background check, they must ride in a cabin seat.