OEM (Original Equipments Manufacturer) is a way to distribute products without accesories. Often these are cheaper ways to get something you want. For example, I purchased an OEM 160 GB Sata II Hard Drive from a site called Newegg.com for $49.99. Sadly, I could not find the retail, but this shows that OEM's are usually cheaper. Be warned though, normally OEM's do not come with cables or manuals.
Overall, I find OEM to be a great way to get stuff, it is cheaper, and works better for people have an understanding of the basic computer structure.