Original Equipment from Manufacturer (OEM) is just that, the equipment. It comes with no accessories and usually no manual or other documentation of any kind. So an OEM copy of Windows would be a disc in a sleeve with a registration number sticker on it. A retail copy comes in a box with an installation guide and whatever else they include. Retail is generally more expensive since it has more stuff. Also, retail is what you'll find at most stores like Best Buy or any retail store (hence 'retail'). You usually can only find OEM stuff online or at specialty shops like those mom and pop computer places.
Most copyrighted software (like Windows) can only be legally installed on one machine at a time. It doesn't matter whether it's OEM or retail. This means that you legally must own a unique copy of Windows for each machine you have it installed on. Also, the retail copy of Windows (or whatever program) should be exactly the same as the OEM copy. The only difference is OEM won't come with any documentation or accessories.
