OEM are for large distributors only, like computer resellers, they only come with a cd and instructionmanual. Retail products are meant for the shops, they are with a box and sometimes some extras.
An example is Windows. When you build a computer and you want to buy parts, then you are licenced to buy an OEM version of Windows (EULA: This product can only be shipped with a computer). When you want to buy only Windows, then you must buy the retail version.
edit: forgot to say: OEM and retail versions of software are exactly the same, the only thing that is different is the box and some extras (like bonus software).