An Operating System is the software that communicates with the hardware of a PC and functions as the intermediary between the installed applications and the hardware, which includes peripherals such as a printer, a computer mouse or a keyboard. When you run an app, it transmits requests to the OS using an API (Application Program Interface) and the contact is done via a command line or a Graphical User Interface (GUI). Similar to a personal computer, hosting servers also use an Operating System which serves as a host for all the software installed on them, which includes script apps or server-side software such as a flash server, a VOIP server, and many others. A virtual machine can be set up on a physical one, so you can install a guest OS on it and have a different system environment in contrast with the physical server host OS.