A guest OS is an operating system that is installed in a virtual machine or disk partition in addition to the host or main OS. In virtualization, a single computer can run more than one OS at the same time. In disk partitioning, a guest OS must be the same as the host OS. In a virtualization solution, a guest OS can be different from the host OS.
A program called Boot Camp allows users of Intel-based Macintosh (Mac) computers to run Windows XP as the guest OS. Once Boot Camp has been installed on the Mac's hard disk, the user can boot the computer using either Mac OS X or Windows XP.