OEM

The automobile wheels from the original vehicle manufactured, are considered to be "OEM" or Original Equipment Manufacturer. These are the wheels that the vehicle's maker installed on the vehicle and it is for this reason they are given the term "OEM". In reality, the vehicle maker (GM, for example) rarely makes anything; they contract with suppliers to manufacture parts to their particular standards of quality, and then they bring these parts together, either for vehicle construction at an assembly plant or into their distribution chain for individual parts sale. Usually (but not always), they are stamped with the vehicle manufacturer's brand-name ("GM", for example) though some manufacturers such as Porsche do not actually have their brand-name applied to their OEM wheels.

Aftermarket

With regard to automobile wheels, "Aftermarket" refers to any part that is not manufactured by or expressly for, the original vehicle manufacturer. This can be something as benign as custom wheels you might see offered in your local speed or wheel shop with a very different look than the vehicle manufacturer had intended, but it may also be something far more innocuous...
There exists a growing industry of small manufacturers of exact replicas of OEM wheels. It amazes me that the automobile manufacturers tolerate this, given that these replicas appear EXACTLY identical from the point of appearance. However, they are most definitely NOT the same. I have inspected these replica wheels, and they are unilaterally of inferior quality to their OEM counterparts. How? QUALITY. It is obvious that the reason for the aftermarket replicas, is to provide what may be construed as the same item at a lower price. But there is generally only one way to make the 'same' item at a significantly lower price, and that is to remove value. These aftermarket replicas are manufactured, either in China or Taiwan, to the very minimal standards to still appear the same as the OEM design. But their failure rate is astronomical. The chrome replica wheels, always begin to flake/peel within a year. I'm in the business and I see these wheels at car dealers after a short amount of time. Chrome replica wheels always being to flake and/or peel with a year, compared with OEM/Factory chrome wheels that remain uniform for 5 or more years (depending on the conditions to which they are exposed). But what is catastrophically worse, is when these aftermarket wheels truly FAIL - as in CRACK. Yes, I have seen on NUMEROUS occasions, where one of these wheels has actually cracked! This is due to the inferior quality of materials/metallurgy of these aftermarket replica wheels. The replica manufacturers seem to answer to nobody due to their foreign manufacture.
People buy aftermarket replica wheels on the internet, and when such a catastrophic failure occurs, they return to the (new car) dealer for warranty replacement. The service advisor will recognize that this is an aftermarket replica wheel, and obviously not only not cover the failure of the wheel, but will not cover any other damage that occurred as a result of the failure. While I personally disagree with the following practice, I've even seen some service managers go so far as to void the remainder of the vehicle's warranty on everything (including unrelated systems), solely due to the use of this type of aftermarket part.
Something to seriously consider, when one is shopping purely based on price.

As a rule and for the above reasons, I generally do not carry nor endorse aftermarket replica wheels.

Aftermarket Chrome-Plated OEM Wheels

For over 20 years now, people have been attracted to CHROME versions of factory wheels. What began in Southern California spread across the country, and became so popular that in the mid 90's the automakers realized they could no longer ignore it, and began themselves to offer factory-chrome wheels. Still, they are not always easy to procure (if the vehicle wasn't originally ordered with them) and also, people don't want to just discard their existing (high-quality) OEM wheels. So an individual with a set of factory/OEM wheels that wanted chrome wheels, had a need that an industry developed to fill: Aftermarket Chrome-Plated OEM Wheels. Chrome-plating wheels is a science, particularly because most wheels are aluminum, and chrome does not adhere to aluminum. Several stages and applications along with preparations along the way, need to occur to ensure a proper chrome-plating. The quality of the finished product is only as good and long-lasting, as the particular company's process. So care and investigation must be taken to ensure that the company that performs this process is reputable.

I will on occasion, sell these Aftermarket Chrome-Plated OEM Wheels. Why? Because some styles of OEM wheels were never offered in chrome and people want that. And because many of the wheels I liquidate, are sold new by some new car dealers on their new vehicles. In the case of my Cadillac wheels - that is exactly where they came from - overstock from a Cadillac dealer's parts department that uses them on new Cadillac's. Because they are sold on new Cadillac's - they have to meet GM standards for quality because when sold on new Cadillac's they are subject to GM's warranty and GM doesn't want warranty claims from known inferior products.

Aftermarket Chrome-Plated OEM Wheels are not something to be feared, so long as due diligence is applied beforehand.

For these reasons, I only use the words "OEM" or "Genuine" (Genuine GM, for example) when the item is as-such.

In summation, OEM automobile wheels are usually of better quality than their aftermarket counterparts, but not always. Oh, OEM wheels are ALWAYS better than their aftermarket replica counterparts. But with regard to truly custom wheels (Asante, BBS, Momo, Speedline, etc.) these products are generally as-good or better than OEM. And priced accordingly.

The old adage holds true, You get what you pay for. And for those who only shop price, Caveat Emptor.