OEM stands for Original Equipment Manufacturer, and is used to describe parts that are made by the manufacturer of your car. OEM parts are also known as Geniune or Factory parts. We recommend OEM!
Aftermarket is used to describe parts that are NOT made by the manufacturer of your car, in other words, they are copies or knock-offs. Insurers have other names to promote them, such as Quality Replacement Parts, or Certified Aftermarket Parts.