Nylon or steel
A standard four-section door rides on ten rollers: one at each end of the three hinge lines, and one in each top and bottom bracket. Each is a wheel on a stem; the stem sits in the hinge or bracket and the wheel runs in the track.
Steel rollers are the cheaper, louder kind, and the plain ones have little in the way of bearings. Nylon rollers with sealed ball bearings run quieter and do not need the track oiled, which is why a quiet-door job is usually a full set of nylon rollers. Stem length and wheel size have to match the hinges and the track, so rollers are matched to the door rather than bought by the bag.