Just out of curiorsity does Windows run inside VirtualBox or does VirtualBox run inside Windows? Or, put another way, does one install VirtualBox on a blank hard disk and then install Windows and Linux etc?
Also, out of curiosity, doesn't the use of VirtualBox (or equivalent) use up a significant amount of the PC's performance before any OS gets a look in?
I have been thinking about something like this but I have never tried it.
The only thing that can be installed on a blank hard drive so that you can boot it is an operating system (because the definition of an operating system is that you can boot it), so your question about installing it on a blank hard drive is equivalent to asking "Is VirtualBox literally an installable operating system like Linux"? Which is an interesting question and the answer is "No". As far as I know neither virtualbox, nor VMWare, nor QEMU nor any number of other other options can literally be an operating system themselves that you can install on a blank hard drive. They are all applications, like Chrome, Firefox.
So. If Firefox exists for a Mac, you can run Firefox on the mac and visit the above link to emulate an Apple 2.
what you install inside the emulated computer is called the guest, as someone stated. The computer you're running the program on is called the host.
So just think of VM's like a Firefox browser, and the emulated computer like my Apple 2 emulated link - except a lot faster, because they're closer to the metal. They can also install drivers that let you copy and paste between the guest OS and the host OS, and so forth.
for example it's a great way to isolate viruses. they don't have any way to "break out" into the rest of your computer, just like the emulated Apple 2 inside Firefox can't write to your hard-drive. it doesn't even know about your hard drive and can't ask about it.