Simple answer:
1. A Retail CPU is sold by AMD to consumers through retailers, for consumer use. They come complete with heatsink and fan.
2. An OEM (original equipment manufacturer) CPU is sold by AMD to system builders (Dell, Gateway, Compaq, etc.) to be incorporated into pre-built computers, which are then sold to consumers.
3. OEM CPU's are not intended by chip makers to be sold to consumers, and they will not be supported by the maker. Chip makers rely on the computer retailer to warranty and support the CPU as part of a pre-build computer.
4. Retailers who deal in OEM's basically get hold of excess chips and sell them at reduced cost because there is no factory support and no heatsink/fan unit. These retailers warranty the CPU's themselves, and set the length. There is no rule on the length of OEM warranties-they generally run from 15 days to 1 year, and are only as good as the companies that offer the warranty.
OEM chips are only justified when the cost is excellent and when you are able to personally observe the chip, such as at a computer show, to hand pick a particular stepping. Retail CPU's hold their value much better, and are easier to resell at higher prices on auction sites, when upgrade time comes.