This term (OEM) is used in many different industries and for many different applications. In general, OEM hardware is boxed as just the hardware alone. It is often sold to PC builders and the like to be used in prebuilt machines. The big differences are the accessories included (no wires, manuals or coolers etc.) and the warranty. OEM usually has a lesser warranty than a boxed retail version.
In general, if you have the other needed items (like a CPU cooler for an OEM CPU or a SATA cable for an OEM hdd) they are a better deal. I usually swap out HW before the shorter warranty is up, anyways.
As stated, you are confusing MS's OEM Windows EULA with the OEM term in general.