An Operating System is the software which communicates with the hardware of a computer and functions as the intermediary between apps and the hardware, including peripherals such as a printer, a 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 via a command line or a Graphical User Interface (GUI). Just like a personal computer, web servers also feature an Operating System which functions as a host for all the software installed on them, including script apps or server-side software such as a flash server, a VOIP server, and so on. 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.