The "VirtualBox" is an application, with whom you can simulate whole computers. You can say it's a computer in the computer.
The application creates a virtual hard disk on which you can install any operating system.
I installed Linux and Windows 7 in my VirtualBox, because I have no disk partition to install these operating systems normaly. And it's to unsecure to install Windows 7 on your normal computer, because the VirtualBox has a big advantage: You have the virtual harddisk and if you have any files which could damage your computer you can delete the virtual hard disk and you're on the safe side and your computer wouldn't be damaged.
So the VirtualBox is a very helpful application if you want to test unsecure operating systems.
Furthermore you can assign your virtual computer different hardware, for example main memory or graphic memory. But playing computer games doesn't work with VirtualBox because you're always running two operating systems to the same time.
Finally, VirtualBox is a good solution for people who want to test Windows 7 or Linux without destroying their whole computer.