A wheel bearing is the part that allows the wheel and tire to spin freely. It is usually made up of two rows of ball bearings surrounded by an inner and outer bearing race — the surfaces that the bearings roll on. The bearing is filled with grease and fitted with seals on both sides to keep the grease from leaking out.
If the bearing grease leaks out, the bearing will overheat and become damaged. A damaged bearing will make a howling noise that increases with speed but will get quieter when you go around a corner. Also, when the vehicle is lifted in the air, the tire will feel loose when moving it up and down or side to side. Extremely worn bearings can cause damage to brake and other suspension components.
Different styles of bearings are used depending on the vehicle. Some vehicles have individual bearings that are pressed into the hub assembly, which is then bolted to the car, others have bearings that are integral parts of the wheel hub assembly.