An Operating System is the software which communicates with the hardware of a computer system and works as the intermediary between the installed applications and the hardware, including peripheral devices 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). Identical to a personal computer, hosting servers also have an Operating System which acts as a host for the software installed on them, including script apps or server-side software such as a flash server, a VOIP server, etc. 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 Operating System.