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