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