OEM Parts (Original Recycled Parts)
This means the part is original, and built by the vehicle manufacturer. Jeep, Dodge, Ford, BMW, Toyota are a couple of OEM brands. Using original used parts are more reliable than aftermarket, they have better fit and finish over aftermarket. New OEM parts are more cost and used OEM parts are usually cheaper or around the same price as aftermarket. You are also doing the environment a favor by re-using a part. You can even use used engines and transmissions.
Aftermarket Parts
When you need to replace the broken screen on you phone, do you buy a factory replacement? or do you go on Amazon and get a aftermarket one?
Most people will go with the cheaper, but still useable aftermarket screen. However does that aftermarket screen work as good or fit as well as the original part? Most likely not; aftermarket parts are not always exact and sometimes need adjustments to make them work properly.
Reference: ChaneysCollision