OEM stands for original equipment manufacturer. These are parts that are supplied by the same manufacturer that made the appliance and are said to be OEM replacements. You should always first look for genuine factory repair parts.

Should an original replacement part be discontinued by the manufacturer that is the time to look for an aftermarket or universal replacement part.

Aftermarket or generic replacement parts may be made by the same company that supplies the originals to appliance manufacturer or they may be produced by a company not affiliated with the appliance manufacturer in any way. In most cases these will be the exact (or very close) fit as the original but their specifications may vary slightly from the originals.