An Operating System is the software which communicates with the hardware of a computer and functions 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 sends requests to the OS through an API (Application Program Interface) and the contact is done via a command line or a Graphical User Interface (GUI). Just like a personal computer, hosting servers also feature an Operating System that functions as a host for all the software installed on them, including script applications or server-side software such as a flash server, a VOIP server, etcetera. A virtual machine can be generated 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.