An Operating System is the software which communicates with the hardware of a computer and functions as the intermediary between apps and the hardware, which includes peripheral devices such as a printer, a computer mouse or a keyboard. When you run an app, it transmits requests to the OS through an API (Application Program Interface) and the contact is done through 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, which includes script applications or server-side software such as a flash server, a VOIP server, and many others. 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.