OEM (Original Equipment Manufacturer) parts are made by or for the carmaker and match factory fit and finish. They cost 20–60% more than equivalent aftermarket parts but tend to fit perfectly and last longer.
Aftermarket can be excellent — brands like Bosch, Denso, NGK, ACDelco, and Brembo make many of the OEM parts themselves. Skip cheap no-name brake parts and ignition components; the savings aren't worth the comeback.