Buying OEM versions of Windows Vista: the facts

Every time there's a product launch, especially for a Windows OS, the question arises about what is OEM and why is it so much less expensive than retail. Well, ARSTechnica has taken some of the guessing (and fun!) out of it & explained a bit in detail what differentiates it. 

OEM editions of Windows don't come in pretty boxes, nor do they include manuals.
Of course, a real "manual" has never shipped with Windows, and chances
are, if you're interested in OEM software, you don't need what passes
for a manual these days, anyway. And the pretty box… well, that's not
really worth much anyway, is it? Rest assured, though: the software on
the disc is the same as that found in any retail edition.

Read the rest here