OEM stands for Original Equipment Manufacturer. This refers to a company that makes parts that are used in another company’s final branded product. A good example is Apple screens. Many Apple screens are manufactured in Samsung factories, then sold to Apple in the form Apple designed.
So, in summary, OEM parts are genuine branded parts that are the same as the ones originally used in the device.
On the other hand, aftermarket parts are made by an outside company that often has no connection to the manufacturer or brand. These companies try to replicate the manufacturer’s products, similar to a generic version of a product at a store. Although aftermarket products are usually cheaper, they are often less functional and may not perform as well or last as long.