An Operating System is the software that interacts with the hardware of a computer and functions as the intermediary between apps and the hardware, which includes peripheral devices such as a printer, a mouse or a keyboard. When you run an app, it transmits requests to the OS using an API (Application Program Interface) and the communication is done via a command line or a Graphical User Interface (GUI). A lot like a personal computer, hosting servers also use an Operating System which functions as a host for all of the software installed on them, including 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 to the physical server host Operating System.