An Operating System is the software that interacts with the hardware of a computer and is the intermediary between all the applications and the hardware, which includes 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, web hosting servers also feature an Operating System which serves as a host for the software installed on them, which includes script applications or server-side software such as a flash server, a VOIP server, and so on. 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 with the physical server host OS.