OEM = Original Equipment Manufacturer
these are system builders typically, like Dell is making a system whichn has ATI card, so ATi wont send dell a fully boxed card, it will only send Dell with the graphic card and its drivers (nothing else, no cables). Dell in this case is called OEM manufacturer, and the cards that are declared as OEM are with card and the driver CD only (no box, cables eytc etc)
if u want to get some other example quoted, like CPU/HD. lemme know although u must have got the idea behind
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"