Generic operating systems?

[R3st!nP3ac3]

ok i went to pricegrabber and searched for windows XP full install. and i saw the lowest price at 165$ which is pretty damn good, then i saw one for 68$ full install.i was like wtf...and it says it was the "generic brand" and said something about OEM. ok how can an OS have a "generic" type? i just dont see how that can work...is it the same exact thing?

[Bad Company]

One will be a boxed retail, the other probably boxed, but it is OEM, which means it is supposed to be bundled with a PC.