If you've opened this book randomly on this page, then you might now know what virtualization actually is. If you've done the normal thing, and started at the beginning, then there's an equally good chance you already understand that you've been using virtualization for almost all of this book.
Virtualization is the act of virtualizing (I know right?) one machine inside another machine. Unlike a container, though, we visualize everything from the USB controllers to the floppy drive (seriously).
This concept is not new, but the technology marches on.
For our examples, you've probably done the same thing I did and resorted to Vagrant with VirtualBox. I chose to go this route because VirtualBox is everywhere, with versions available for macOS, Linux, and Windows (along with others!). This has great advantages, but also disadvantages...