OEM stand for Original Equipment Manufacturer.
The difference between "Retail and "OEM" is like:
-Retail package will included drive, mounting brackets, SATA cables, and maybe some software, in a brand name box.
-OEM will just included the drive, may not even be in a box. So you have to get the cables, mounting hardware yourself. This adds to the cost of the OEM drive, but some already have all that stuff in the PC case, and will just swap out the old for new drive.
I bought Windows 7 OEM, so all I got was a install disk in a CD paper sleeve. No book/manual or packaging. I do get support, though.