O Origional
E Equipment
M Manufacturer
OEM parts are made primarily for the large package computer companies (gateway, dell, etc..)who buy from the manufacturer in bulk. They have no need for the retail package since the end consumer will never see it. As far as the HS/Fans, they generally have a seperate bulk contract for those also. They get special prices & RMA deals since they work with such quanities, so the retail warrenty is not neede by them also. Some large wholesalers & computer stores also will get OEM parts. The physical item is identical (Intel prints a different code by the serial # for OEM chips, not sure about AMD) to the retail versions, less the retail package.
One work of warning on some places that sell OEM chips. Some places will see how well each chip (or batch of chips) overclocks & set those aside. If you are an overclocker, you may be getting a chip that will not be a good overclocker. Where as a retail chip will be sealed. Not all places do this. I have an OEM PIII 700 that does 980 easily.