An Operating System is the software which interacts with the hardware of a computer and works as the intermediary between all the applications and the hardware, which includes peripherals such as a printer, a mouse or a keyboard. When you run software, it transmits requests to the OS using an API (Application Program Interface) and the interaction is done through a command line or a Graphical User Interface (GUI). Identical to a personal computer, web servers also feature an Operating System that functions as a host for all the software installed on them, which includes 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 could install a guest OS on it and have a different system environment compared to the physical server host OS.