VirtualBox is described as a 'general-purpose full virtualizer for x86 hardware' similiar to VMWare.
What this means is you can run another, or multiple, operating systems as guests on one host computer. It is designed for use on servers, desktops or embedded in devices like mobile phones and PDA's embedded use. VirtualBox is also to date the only professional virtualiser available as open source released under the GPL license.
VirtualBox allows you to test out operating systems by installing into a virtual drive in your existing operating system. Currently their are editions for Windows, Linux and an OS X Intel version is being tested.
An example of this would be to test out Linux of you have never tried it. You could download a distribution of Linux and install it inside a window in Windows! Alternatively if you have switched to Linux and still need to run some Windows based software then VirtualBox will let you do this.
Currently guest operating systems that are able to be installed and include Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.
Another cool feature of VirtualBox is the ability to run images made for VMWare's VMPlayer. You can find images for various operating systems over at the VMWare Technology Network website
VirtualBox is actually distributed as two versions. There is the open source GPL version and the Personal Use and Evaluation License (PUEL) edition. The differences are quite simply that there are certain features excluded from the GPL version such as USB support and shared folder support.
If you require the extra features and are using VirtualBox for personal use, at home for isntance, then the PUEL edition should suit you fine. However if you are in a business then you would need to purchase a license or use the GPL'd version.
