An Operating System is the software which communicates with the hardware of a computer system and functions as the intermediary between applications and the hardware, which includes peripherals 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 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 which functions as a host for all of the software installed on them, which includes script applications 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 compared to the physical server host OS.