These are parts that are made by the original equipment manufacturer (which basically means that these are parts that are made by the same company that makes or manufactures the vehicle).
Aftermarket or Replacement parts that are not made by the original equipment manufacturer and are produced by a different parts company. They are often designed to be compatible with as many makes and models as possible.
Exjapan parts are primarily OEM used parts taken from other cars that have been decomissioned in one way or another. These may include body parts, electrical parts, rims, some suspension components etc.
Question: Which one should I choose for use in my car?
This is a question that has haunted many a car owners. To answer this question the team handling your car should be competent enough to handle the vehicle by understanding the vehcile application in terms of the kind of parts it uses. This is key in ensuring a seamless trouble free and overall pleasant experience with the car.
New OEM Parts cost considerably more than the Aftermarket parts, while in some cases may Ex-Japan parts cost more than new aftermarket parts. With that said some of the aftermarket parts are also of superb quality almost or matching the quality of the OEM parts. However it's also important to note that some extremely cheap parts are detrimental to your vehcile in that they are substandard in quality and will not serve you well thus lead to frustration.
OEM is the best way to go but there will be a cost implication to this choice thus in our experience we've observed that the pricing of the parts is usually the determinant factor for the car owner.