"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Pontiac Trans Sport, if you buy accessories for your car that are not actually made by Pontiac, then those are termed "aftermarket". Many people use the term "Trans Sport aftermarket accessories" specifically when referring to custom accessories for the Trans Sport, like performance accessories or custom appearance accessories. But even if you just replace your Trans Sport brake pads with OEM-grade factory-style brake pads that are not made by Pontiac, those too are aftermarket Trans Sport accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".