OEM is Original Equipment Manufacturer...it's what comes as original equipment DIRECT from the factory...that's how the model is ALWAYS packaged and what's included in the box....for some items like video cards...the manufacturer card that is sold is also modified by the VENDORS...like MSI, ASUS, Sparkle etc....they take OEM equipment, and modify it with extra items like better fans, heat sink shrouds, or they even order from the manufacturer with differing specifications than the stock part...the boards are made a bit different, with differing components... much like buying a stock mustang or buying a SHELBY mustang...or a Eddie Bauer Edition Ford Bronco...the stock comes from the manufacture but there are special edition models available as well....IE...OEM vs Special Edition whatever!!!
Sorry for car comparison...but concept is easier to understand that way or at least easier to explain that way.
Respectfully,
tekman42