OEM means Original Equipment Manufacturer.
That simply means that it is the version sold to equitment manufacturers like Dell, HP, etc.
So OEM is usually the "preinstalled" version of windows that you get when you buy a new computer with OS already included.
And the thing about OEMs, the last time I checked, was that it comes with no technical support from microsoft and OEM's might be less expensive than retail because OEM's might not be packaged with the same stuff that retails come with, like the pretty boxes and things like that.
However, if you get OEM windows xp and retail windows xp, you will see absolutely no difference in the main product. So windows is still windows guaranteed.