An Operating System is the software that communicates with the hardware of a computer and works as the intermediary between the installed applications and the hardware, including peripherals such as a printer, a mouse or a keyboard. When you run software, it transfers requests to the OS through an API (Application Program Interface) and the communication is done via a command line or a Graphical User Interface (GUI). Similar to a personal computer, web servers also use an Operating System that functions as a host for all 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 created on a physical one, so you could install a guest OS on it and have a different system environment compared to the physical server host Operating System.