Parts and componets that are offered to the consumer other than those from the original factory or dealer. For the automoive market, they are avalilable at any auto parts store ie, Advance Auto, Autozone etc. Anything other than original equipment manufacture (OEM)
There are plenty of good after market parts suppliers that provide parts at an equal or even higher quality than OEM parts. The advantage of OEM parts is that the parts you are replacing are exact matches to the ones you are putting in new. With OEM there is no question about the fit or match of the part you are putting in. Sometimes, aftermarket parts may need adapters or modification to fit properly.