Clearance between pad and rotor?

This is probably a stupid question, but is there supposed to be some space between the brake pads and the rotors on disc brakes? I've never noticed on any other vehicle, but my truck pads are constantly rubbing my rotors. The front wheels don't spin as freely as the rears, which are drum brakes. If they are supposed to rub, how much rubbing is too much?

Your wheels should spin freely. Your calipers might need to be rebuilt to eliminate the rubbing.

Is there not any way to adjust it?

No adjustments.