Guest operating system
Guest operating System (GuestOS) is a second operating system that is installed inside the host operating system. It is typically installed as a virtual machine.
Compatibility
Guest operating systems share the devices and memory of their hosts. Some systems can simultaneously run 32 bit and 64 bit guests.
Examples
Microsoft Windows can be installed inside a Linux operating system as a virtual machine, Linux can be installed inside a Windows machine with the help technology such as of VirtualBox.