innotek VirtualBox is a family of virtual machine products targeting desktop computers, enterprise servers and embedded systems. Due to its modular architecture, VirtualBox can be deployed in any environment where x86 systems are to be virtualized on x86 systems. (With "x86", we are referring to 32-bit CPUs from AMD and Intel as well as compatible CPUs from other vendors, plus 64-bit CPUs in 32-bit mode.)
VirtualBox 1.4 added support for Linux 64-bit hosts and Mac OS X hosts; with version 1.5, support for 64-bit Windows hosts ,seamless windows and virtual serial ports were added.
With VirtualBox, you can run unmodified operating systems -- including all of the software that is installed on them -- directly on top of your existing operating system, in a special environment that is called a "virtual machine". Your physical computer is then usually called the "host", while the virtual machine is often called a "guest".