An Operating System is the software that communicates with the hardware of a PC and functions as the intermediary between applications and the hardware, which includes peripherals such as a printer, a mouse or a keyboard. When you run an app, it sends 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). Identical to a personal computer, web servers also feature an Operating System which 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, etcetera. A virtual machine can be set up on a physical one, so it is possible to install a guest OS on it and have a different system environment compared to the physical server host OS.