Windows 8 installation method

Okay, the most economical way to get Windows 8 is through the digital Microsoft upgrade for $40. But what would I get? Do I get an installation ISO? A file to create a USB disk with? Or an installation that required to be run under an existing Windows install? Will the serial for that Windows 8 be locked with the Windows 7/Vista/XP it was bought from? Can I buy Win 8 this way and use it to upgrade a different Windows PC?

This is what I hate about Microsoft's way of having different type of licenses between upgrade and full. There's no clear way on how to do things anymore.