Difference between OEM an Retail?

OEM can only be installed on one computer (tied to the motherboard). Once it is installed it can't be installed with another motherboard...legally.

Retail can be used over multiple times on different computers and motherboards as many times as you like. But it can only be installed on one at a time.

Retail can be installed on multiple machines (one at a time though.) OEM can only be installed on one motherboard.

So it like those pre built machine OS? Which is "Tattoo" right?

OEM means that the licence holder is the motherboard and not the person. This means that you cannot upgrade your motherboard but can upgrade your other parts, due to you mobo being the key holder

Retail is more expensive but has no restrictions, you can upgrade your mobo as many times as he person is the licence holder

retail comes with the hsf and i think a longer warranty (3yrs vs 1).