An Operating System is the software which interacts with the hardware of a computer system and functions as the intermediary between all the applications and the hardware, including peripheral devices 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 interaction 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 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 many others. A virtual machine can be generated on a physical one, so you'll be able to install a guest OS on it and have a different system environment compared to the physical server host Operating System.