Virtualization technology allows a computer to run more than a single OS at the same time. A guest OS on a virtual machine can be different from the host OS while a guest OS on a partitioned disk must be the same be the same as the host OS. For example, if the host OS is running Windows, then any guest OSes on a portioned disk must also run windows. In a virtualized environment, the guest OS can be different than the host OS. A guest OS is required before a virtual machine can be deployed.
Unlike a host OS, which is installed on a computer and interacts with underlying hardware, a guest OS resides on a virtual machine. Although a guest operating system can use some of the host operating system's resources, the two are entirely separate. A guest OS hosted on a virtual machine can be used for testing without having an impact on anything outside that VM.
Guest OSes are not always an alternative to the host OS. A Type 1 hypervisor (bare-metal hypervisor) runs directly on the hardware of a server, which takes the place of the host OS. The Type 1 hypervisor can create virtual machines, which can run guest OSes. A physical server can have multiple virtual machines and each VM can run its own guest OS. One guest OS can run Linux while another could run Windows.
An example of a guest OS would be running Windows Server 2012 as the operating system in a virtual machine created by the VMware ESXi hypervisor. Another example would be Boot Camp, which allows Macintosh users to run a Windows OS as the guest OS within a virtual machine on their Macintosh.
Guest operating systems can be of great benefit to administrators. Administrators are able to run programs and applications that aren't compatible with the host OS on a guest operating system. Admins can also run more than one application that requires different operating systems on the same physical hardware.