An Operating System is the software that communicates with the hardware of a computer and works as the intermediary between apps and the hardware, which includes peripheral devices such as a printer, a mouse or a keyboard. When you run an application, 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). Similar to a personal computer, web servers also have an Operating System that functions as a host for 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 generated on a physical one, so you will be able to install a guest OS on it and have a different system environment in contrast with the physical server host Operating System.