Where do I find the UPC thingy?

I got 2 rebates with my WD 300GB hard drive I got from frys, I have to send them in seprately because they use diferent addresses (they differ by one number, for the rebate code) and it requires me to send in the purchase reciept, i have one for each rebate, and a UPC/serial number. How do I get this, where si it, and what is it?

It is the bar code that is on the packaging.

its the bar code...