"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Saturn Sky, if you buy accessories for your car that are not actually made by Saturn, then those are termed "aftermarket". Many people use the term "Sky aftermarket accessories" specifically when referring to custom accessories for the Sky, like performance accessories or custom appearance accessories. But even if you just replace your Sky brake pads with OEM-grade factory-style brake pads that are not made by Saturn, those too are aftermarket Sky 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".