Original parts are produced by the original equipment manufacturer (OEM). As OEM parts are often sold under other brand names, the name on an original part may be different from the name on the part that it replaces. Also, as OEM parts are often sold under several brand names, they may come in unbranded boxes when purchased directly from the OEM.
Aftermarket parts are copies of the OEM part. These parts are often made to be cheaper replacements for the OEM part, but sometimes they replace an OEM part that are out of production. Also, some aftermarket parts are produced to improve on the original OEM part and are therefore more expensive.