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