7 Home Premium is a 32 and 64 bit package, (2 discs). It is for the do it yourself person that builds their own. It can be transferred to another computer limited times. Example: I had one that was used every day, 3 years, finally became unusable. The next I built and used the OS from the latter. OEM: These are what retailers and builders use in their products, and add other software to it. They also have to provide support for the systems, example Dell. The OEM is "married" to the computer it is installed in, used only for that system and will not be genuine in any other. If it is installed in any other, Microsoft will not support it. IE. no up dates , nothing. You are also supposed to have a builders certificate to purchase this. Why this package is available to the public I do not know, with the restrictions it has. The price as you can see is taller for the full version.