VirtualBox is a virtualization software package, meaning that it allows you to run other operating systems inside of your current OS. This works by showing the OS is a window on your screen.
When I first heard about this I though, why would use this, it is bound be slow and error prone like other emulators. And besides I can just dual boot, all but the most ancient operating systems.
Having now tried VirtualBox I see the appeal. It was free, fast, and easy to install. It was fast and easy to setup the other operating systems, with the easy to follow setup wizard. and most of all it did not slow down my system at all when running them, even when more then one OS was open at the same time. And best of all, it just works.
The possibilities for testing code or running any app you want are endless. Have an application that you want to try that does not run on any of your currently installed operating systems, virtualization might be the answer for you. Want to test your code on machines that your users are likely to use, you can now do it without leaving your chair of even changing keyboards. Want to use a program without risk of compromising your computer, virtualization is the ultimate sandbox.