OE: Original Equipment: This part is either made by the car manufacture or is made by an automotive part supplier and is branded with the car manufactures logo and/or in the car manufactures box.
OEM Original Equipment Manufacture
OES Original Equipment Supplier
Car manufacturers do not make all their parts that they put on their vehicles during assembly or repair, they contract out to auto part manufactures to make parts for them. For the most part the car manufacturer makes the body, frame and major engine components the rest they 'farm out' to OEM/OES manufacturers. The car manufacturer provides the specifications to the OEM/OES manufactures for the parts they need. The OEM/OES manufactures the part to these specs, adds a logo and ships it to the car manufacture.
Bosch, Bilstien, Boge, Beru, Mann, ATE, to name a few, are all OEM or OES suppliers to the car manufacturers. They make parts from spark plugs to exhaust parts. The difference between OE and OEM/OES is mainly the OEM/OES usually don't have the car manufactures logo, but they are the same exact part. Sometimes the logo is ground off the part by the OEM/OES company so as not to affect there contract with the car manufacture. Same part coming off the same assembly line as the OE part does.
The OEM/OES parts are less expensive because they do not go through the car manufactures part system. Every time an OE part goes through a depot, warehouse, dealer, there is a little more money added to the cost of the part. This is the major reason that OE parts cost more. OEM/OES do not go through this procedure, our buyers get them directly from the manufactures, keeping the prices down.
Aftermarket: aftermarket parts are just that, aftermarket. They are not made by the car manufacturers. They can be made by one of the original equipment manufacturer companies or by a completely different manufacturer. The main difference is they are not made completely to the car manufacture specifications. This is not always bad. One example is Bilstien. They are an OEM/OES supplier, but they offer aftermarket parts also. There HD struts/shocks are original equipment, but their Sport struts/shocks and suspension kits are not made to OE specifications, better but not OE, so it's now an aftermarket part. Another example is the Stewart EMP BMW water pumps. Stewart EMP is NOT an OEM/OES manufacturer but the pump they make is better and stronger than OE. It is an aftermarket part but a better part altogether.
But still another example of an aftermarket BAD part is a counterfeit part. Counterfeit part manufacturers use backward engineering to get the specifications of the part without paying the car manufacturer. And almost all the time these parts are made with very low quality components. Sometimes these parts are very hard to identify because a lot of emphasis is put on the look and feel of the part and not what it is made from. One way to identify an counterfeit part is its unbelievable low price, the old adage you get what you pay for fits here. The best way to avoid these cheap low quality parts is to purchase your parts from a reliable source, one that offers a warranty and return policy.
Almost all Performance parts are aftermarket parts, again be careful with these parts also. Buy from a reputable supplier or/or manufacturer.