OE and OEM products are products from manufacturers who produce the original factory Genuine parts for the car. For example, BMW doesn't manufacture and produce a control arm - Lemforder makes it for BMW and engraves the BMW logo during production. Buying directly from Lemforder instead of the BMW dealership means that you get the same quality part for a lot less cost. We typically recommend the OE/OEM alternative.
Comprehensive comparisons and full details, are HERE: