Aftermarket parts are just the opposite of OEM parts. These parts are produced by companies other than the original manufacturer but are made to fit and perform as well as the original. Aftermarket companies buy the rights to reproduce these parts and supply them to the same wholesale distributors as the OEM parts. These parts are generally produced with the same machinery and materials but because they were not designed in the same manner the part may have minor differences in appearance and feel. Some aftermarket companies, upon getting the right to produce a part, may indeed find a way to improve a part by redesigning it. This results in a better part and quite possibly a less expensive one. There are a variety of high quality aftermarket parts that can be purchased and live up to all the expectations of the original at a much lower price.
In both cases, OEM and aftermarket parts, the part should serve its purpose and perform the function it was designed for. As the car owner it is your choice that ultimately decides which part is chosen. By knowing these few things you can more easily determine which is best for you and your car. Remember in most cases buying a part from the carmaker or dealer does not mean you get a better quality part. The same part can be purchased at a fraction of the cost still give you that same piece of mind feeling.