If you mean what's the difference between OEM and Retail versions, then OEM versions are tied to the first system (motherboard) you install the OS onto. It cannot be transferred to any other system. Additionally, the OEM version has no official Microsoft Technical Support.
Retail versions can be moved from one system to another (but can only be installed on one system at a time) and you do receive technical support from Microsoft if needed.
Functionally, there is no difference between the two.
-Wolf sends