A hubcap is a plastic cover that goes over a steel rim, like what you see on lower end vehicles. To get technical, a rim is only the side of a wheel (without a hub cap). Normally the entire wheel is referred to as the rim.

Most aftermarket wheels are manufactured with a center hole that will fit a wide range of vehicles. The hub-centric ring is used to fill any gap that may exist between this hole and the vehicle's hub, thus centering the wheel on the axle. If you have vibrations after installing new wheels, chances are hub-centric rings were not used.

In general, Alloy rims are made up of alloys of aluminum or magnesium. And, steel rims are made up of rectangular steel sheet metal.