An Operating System is the software that interacts with the hardware of a PC and serves as the intermediary between the installed applications and the hardware, including peripherals such as a printer, a mouse or a keyboard. When you run an app, it sends requests to the OS using an API (Application Program Interface) and the communication is done via a command line or a Graphical User Interface (GUI). Identical to a personal computer, web servers also have an Operating System which functions as a host for any software installed on them, including script applications or server-side software such as a flash server, a VOIP server, and so on. A virtual machine can be created 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.