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