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