VirtualBox is an X86 virtualization software developen by Sun, distributed under two license, PUEL (Personal Use and Evaluation License) and GNU/GPL license.
Wikipedia said, VirtualBox is an x86 virtualization software package, originally created by German software company innotek, now developed by Sun Microsystems as part of its Sun xVM virtualization platform. It is installed on an existing host operating system; within this application, additional operating systems, each known as a Guest OS, can be loaded and run, each with its own virtual environment.
Supported host operating systems include Linux, Mac OS X, OS/2 Warp, Windows XP, Windows Vista, and Solaris; there is also experimental port to FreeBSD. Supported guest operating systems include various versions of DragonFlyBSD, FreeBSD, Linux, OpenBSD, OS/2 Warp, Windows, Solaris, Haiku, Syllable, ReactOS and SkyOS. The latest version also supports Windows 7 beta and release candidate 1.