bascially OEM mean it comming stright from the manufactor. This mean it wont come in a box and it might not come with any wires. Not really a big deal I buy all my cd drives and harddrives OEM usually. It will basically get mailed to you wrapped in bubble wrap instead of in a box specfically made for it.
The drives themselves between OEM and RETAIL or identicle.
They are the same drive. As mgoldb2 said, OEM is just the product, nothing else. You will find retail versions that have the manuals and warrantys and whatever else comes with it at a, you guessed it, retail store like Best Buy.