An Operating System is the software which interacts with the hardware of a computer system and serves as the intermediary between applications and the hardware, which includes peripheral devices such as a printer, a computer mouse or a keyboard. When you run an app, it transfers requests to the OS through an API (Application Program Interface) and the interaction is done via a command line or a Graphical User Interface (GUI). Just like a personal computer, web servers also feature an Operating System that serves 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 you could install a guest OS on it and have a different system environment in contrast to the physical server host Operating System.