An Operating System is the software that communicates with the hardware of a PC and functions as the intermediary between all the applications and the hardware, which includes peripheral devices such as a printer, a computer mouse or a keyboard. When you run an application, it transfers 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). Just like a personal computer, web servers also have an Operating System which acts as a host for 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 set up on a physical one, so you can install a guest OS on it and have a different system environment in contrast with the physical server host Operating System.