An Operating System is the software which communicates with the hardware of a computer and functions as the intermediary between applications and the hardware, including peripherals such as a printer, a computer mouse or a keyboard. When you run an app, it transfers 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 servers also feature an Operating System that 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, etcetera. A virtual machine can be set up on a physical one, so you will be able to install a guest OS on it and have a different system environment in contrast to the physical server host OS.