For a passive buyer, these things are but ordinary terms used in the automotive market but for someone meticulous and who wants the best for his auto, these things matter considerably. Deciding which among these to purchase is just like deciding what car to buy.
O.E.M. stands for Original Equipment Manufactured. This means that OEM Ford parts are manufactured by Ford itself, Chevrolet parts are manufactured by Chevrolet, Toyota parts by Toyota, BMW parts by BMW and so on. The terms O.E.S. and OE are also used; these mean Original Equipment Supplied and Original Equipment, respectively. While in many cases, OEM and OES mean the same, OE is more general referring to any part that came as original equipment on the car. Some of OE car parts and components are not actually made by the car manufacturer but are purchased and assembled by the automakers to create a vehicle.
Those referred to as aftermarket auto parts are not made by the original car manufacturer; furthermore, they are bought and added to the vehicle only at the dealership or after the vehicle left the dealership. In terms of design and function, aftermarket products are almost the same as the stock auto...