Original Equipment Manufacturer, ie, a company that makes a product
branded with their own name, that contains parts from other companies.
[Like Dell, Gateway, etc].
Hence an "OEM Harddrive" is a bare drive intended for wholesale to an OEM
who will incorporate it into their product, and as-such it comes without
any cables or instructions (or even a box) since the OEM doesn't need or
want to pay for that stuff. A "Retail Harddrive" is the same drive
packaged in a pretty box w/ instructions, cables, software, and even
brackets and screws, plus a tech support 800 # and a warrenty card,
intended for display on a store's shelf and eventual sale to an end-user.
All those extras make it cost more.