If you have a computer that came with the Professional edition of Windows 8, you have downgrade rights. This means that you can install Windows 7 over Windows 8 without needing an additional license key.
These downgrade rights are intended for businesses, which is why you can only do this if you buy a PC that comes with Windows 8 Professional. You can’t just upgrade your laptop to Windows 8 Professional and then exercise downgrade rights — you can only do it if your PC originally came with the Professional edition. This effectively locks most home users out, leaving the option only available to businesses.