An Operating System is the software that interacts with the hardware of a PC and serves as the intermediary between all the applications and the hardware, including peripheral devices such as a printer, a computer mouse or a keyboard. When you run an app, it transfers requests to the OS using 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 use an Operating System that acts as a host for all of the software installed on them, which includes 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 will be able to install a guest OS on it and have a different system environment in contrast with the physical server host OS.