An Operating System is the software that communicates with the hardware of a computer and serves as the intermediary between applications and the hardware, including peripherals such as a printer, a computer mouse or a keyboard. When you run software, it transfers requests to the OS using an API (Application Program Interface) and the interaction is done via a command line or a Graphical User Interface (GUI). Identical to a personal computer, web hosting servers also use an Operating System which serves as a host for all 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 it is possible to install a guest OS on it and have a different system environment in contrast to the physical server host Operating System.