These are terms you may hear often where talk of auto body repairs are concerned. When speaking of a “genuine” part, the distinction is that the part is genuine to the manufacturer that made the part and it’s in the manufacturer’s original packaging.
“OEM” stands for “original equipment manufacturer.” This means that the part was made for that vehicle and is designated with the specifications of the manufacturer. However, the part may have been made by an external company.
“Aftermarket” refers to parts that were made by a completely different manufacturer. They might have close to the same specifications of the manufacturer or could be near perfect. These are parts that are often customized or cost less than the manufacturer but come with the risk that they won’t work as well as original or genuine parts. Aftermarket parts often void any warranty from the original manufacturer.