Isn't it neat how computers can completely simulate other computers purely via software? (Background). Well, right now I'm reading a book that comes with a Linux Live CD containing relevant source code -- basically, a CD that you can boot from to see what it's like to use the operating system without interfering with the one you currently have. (Not to mention the benefit of having the exact same environment as the author so you can make sure It Works.) And that's how Live CDs are typically run: from bootup.
But with emulation, you don't need to reboot your whole computer just to get that Linux (or whatever OS) experience! You can set up a "sandbox" environment, or virtual machine that "pretends to be a computer". The software allocates a portion of your computing resources that you specify (disk space, RAM, etc.), and you just run the Live CD through that "pretend computer", freely switching from the window containing that virtual machine, to your web browser and whatnot. Again, it's without the hassle of rebooting every time you want to switch between that and the cute cat video you were watching.
So there I am -- I've got the virtual machine running a "sample" of an operating system off a Live CD, no need to reboot my "real" machine. But it gets better! I can go one step further and tell my pretend computer, "You know what? Let's go all the way. I want the full operating system -- not just the "sample" -- installed on your pretend hardware!" And then it dutifully runs through all the screens you would normally see when installing a Linux distro as your operating system, seizing control of the sandboxed software-that-thinks-it's-hardware, for a full wipe of the, um, non-OS you had there before.
Sorry, I don't know why ... I just find this all so hilarious ... virtualizing the use of a "sample" operating system before I install it on its virtual hardware.
(For those who are curious, the software I'm using is Oracle VM VirtualBox, available free for (IIUC) personal non-commercial use. I learned about it from LessWrong.com's failed attempts to teach others how to play with the site's code.)