There is really no difference between them, except the fact that it will ONLY work on the motherboard you install it on, if you want a new motherboard, you must buy a new licence.
These are the differences:
- OEM versions do not offer any free Microsoft direct support from Microsoft support personnel
- OEM licenses are tied to the very first computer you install and activate it on
- OEM versions allow all hardware upgrades except for an upgrade to a different model motherboard
- OEM versions cannot be used to directly upgrade from an older Windows operating system