Original Equipment Manufacture. These are the components bought by a systems assembler that will warrant the finished product. If you buy the components individually, you are assuming the warranty liability. Other names for OEM are White Box, Brown Box. Ergo, no fancy retail packaging, included extras and no retail warranty.
I just ordered an OEM SCSI host controller. It will have no included software, cables or anything else. I think I get a 30 day warranty if it is Dead On Arrival.
OEM Microsoft products are not supported by Microsoft. You have to buy the retail versions to get Microsoft support with the purchase.