There are lots of parts available for the car. These parts are categorized in two: the aftermarket and the OEM or original equipment manufacturer.
Aftermarket parts are the parts that are made from other manufacturer. These parts usually tend to have same material and purpose. They may have differences in how they look and feel. Aftermarket parts usually made to improve or enhance the performance of the car. You can choose from a variety of parts from different manufacturer. Compare to the OEM parts also know as stock parts.
OEM parts are the parts that is made from the manufacturer of the car. These parts are much expensive compare to aftermarket. It is easy to choose because it is the recommended part of the manufacturer. When talking about performance, your car stay in stock mode when you use OEM.
Some say OEM is good and some say aftermarket is good. But it is your choice on what is the best for your car.