An Operating System is the software which interacts with the hardware of a PC and functions as the intermediary between apps and the hardware, which includes peripherals such as a printer, a mouse or a keyboard. When you run an application, it transfers 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). Identical to a personal computer, web servers also have an Operating System that serves 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, etcetera. A virtual machine can be set up on a physical one, so you'll be able to install a guest OS on it and have a different system environment in contrast with the physical server host OS.