VirtualBox is a piece of software that uses virtualisation to simulate a PC. With it you can run Windows, Open BSD or even Linux from your Debian system. Since it also runs on Windows and Mac OS, you can use it to run Debian from that other non-free OS. Note however that it only works on x86 and x86_64 hosts.
You can do all this in a nice and hassle free GUI, driven by menus or application wizards. Here you can see the mentioned "KDE Four Live" CD booted up and ready within VirtualBox.