An Operating System is the software that communicates with the hardware of a computer and is the intermediary between the installed applications and the hardware, which includes peripherals such as a printer, a computer mouse or a keyboard. When you run an application, it transmits requests to the OS through an API (Application Program Interface) and the communication is done through a command line or a Graphical User Interface (GUI). Similar to a personal computer, web servers also feature an Operating System which serves 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, and so on. A virtual machine can be set up on a physical one, so you will be able to install a guest OS on it and have a different system environment in contrast to the physical server host OS.