OEM is an acronym “Original Equipment Manufacturer”

“OEM”and Aftermarket Parts should not be confused. Most car manufacturers find it impossible to actually manufacture all the various parts in their cars and thus use independent outside manufacturers.
These independently made parts are what you will find boxed by car manufacturer as their own and sold in your local dealer. 
After a period of time as controlled by the car manufacturer these same independent manufacturers are also allowed to sell the exact same part into various distribution channels boxed in their own typically also well known brand. These parts are made to the exact original equipment specifications and usually sold at up to 70% what you would find at your local dealer. 
An OEM part is one that is either supplied directly from the car manufacturer or is made by the same outside manufacturer as supplied to a car manufacturer.

Aftermarket parts are made by companies that may purchase the rights to make a product, or reverse engineer a product and thus are built to fit and perform as well as the original but were never actually supplied to the manufacturer. While these parts are typically made with the same type of machines and with the same type materials, slight visual differences are often evident. Some aftermarket companies also continue to engineer a product and thus can often make a product that is actually better or longer lasting than the originals. And often times they are much cheaper.