What is the difference between Windows 7 and Windows 7 Upgrade?

I am using Windows XP and I am considering getting Windows 7. I have been shopping online and I have found Windows 7 and Windows 7 Upgrade. When I think of an upgrade, I think of it as something to improve something that I already have. Is this the case with Windows 7 Upgrade? Is it an improvement on Windows 7? If so, why hasn’t the upgrade been integrated with the Windows 7 software?