"OEM" stands for Original Equipment Manufacturer.

This means that the items that have the letters OEM in their description generally cannot be sold on their own.
OEM items are not suppled with manuals, cables, documentation, retail boxes and quite often no software or drivers.

You cannot order a Microsoft item that says OEM in it's decription without purchasing other computer hardware. or a computer system.

For Microsoft Products;
To quote Microsoft's OEM agreeement "You may only distribute such hardware with a fully assembled computer system or other non-Microsoft computer hardware components"