OEM, i.e., what big companies would buy to make systems to sell to you. Also called "white box", you don't get the retail box. In some cases (video cards) OEM cards are made with sub-standard parts. In the case of CPUs, they are exactly the same chip, but you don't get a 3-year warranty, the retail box, or the fan/heatsink. All you get is the chip and a 10-30 day warranty (depending on the seller).
That said, I always buy OEM because it's cheaper, I don't care about the box, I usually use a better HSF for overclocking anyhow, and the "extended" warranty is useless to me - if it works when I pop it in my system, I'm happy. These chips don't just die at random.