Garage door rollers are actually the wheels, which carry the door and sit in tracks using them as guides to move up and down. There are usually three rollers in each side and they are connected to the door with hinges, except from the two bottom ones that are connected with brackets. These are the most difficult ones to replace since the spring cables are attached to these same brackets and their removal might result to cable snapping and injuries.
Rollers are usually made of steel and might have multiple ball bearings so that they can move without making loud noises. They also come out in stainless steel for high moisture areas. Nylon rollers are ideal for quiet operation but not for extremely heavy garage doors. When it's time to have the rollers replaced, the diameter and length of their stem must be measured and they are always picked in accordance with the garage door's weight. It is important to consult with a professional, as the parts and materials must be picked accordingly to the door as well as to the weather and climate conditions.