Aftermarket appliance parts are generic replacement parts usually made when the manufacturer no longer can supply the original. Often called copycat parts, they are, in most cases, an exact fit for the original. Specifications could vary slightly, but they are generally a close fit.
Most aftermarket parts are referred to as "functional" or "fast moving". These include belts, switches, motors and controls. They may be manufactured by the same company that makes the original part or, sometimes, by a company not associated in any way with appliance makers. Aftermarket parts are popular because they offer a more affordable option for appliance repairs.