VirtualBox helps us to load and run an operating system on an existing operating system. The operating system on which VirtualBox is installed is called the host OS, and the OS running on the host with the help of VirtualBox is called the guest.
For example, consider a machine with the Windows XP operating system loaded on it, VirtualBox can be installed on this, and another OS, such as Fedora or Solaris can be loaded via the VirtualBox. Here Windows XP is the host OS and the other OS (Fedora, Solaris) becomes the guest.
A free copy of VirtualBox can be downloaded from here.