OEM - Definition
OEM stands for Original Equipment Manufacturer. That means that the part you are purchasing is made by the original company that made it, not an aftermarket manufacturer.
OEM - The Difference
You may see a lot of companies advertising their parts as "OE Equivalent", "OEM+", or "Meets or exceeds OEM Specifications". Many times this can be true, but unless it's a good brand name like Katzkin, Moog, or Bosch, odds are that the statement is purely exaggeration and a marketing ploy. When it comes to aesthetic parts like tail lights, or even seats, aftermarket parts often times aren't DOT approved and they haven't gone through the rigorous safety and quality testing that's required of the OE Manufacturers. A dead giveaway of this is when the part has a disclaimer that says "For Offroad Use Only". This absolves the manufacturer of any liability in case of an "on-road" accident.