This is the software publisher's description.
Powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See "About VirtualBox" for an introduction; see "innotek" for more about our company.
Presently, VirtualBox runs on Windows, Linux, Macintosh and OpenSolaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.
VirtualBox is being actively developed with frequent releases and has an ever growing list of features, supported guest operating systems and platforms it runs on. VirtualBox is a community effort backed by a dedicated company: everyone is encouraged to contribute while innotek ensures the product always meets professional quality criteria.
On the VirtualBox site, you can find sources, binaries, documentation and other resources for VirtualBox.
VirtualBox 4.1 is a major upgrade. You can now clone your virtual machine (ideal for creating a backup version of your new VM), support for 1TB of memory under a 64-bit operating system, improved UI and more.