An Operating System is the software that communicates with the hardware of a computer system and functions as the intermediary between all the applications and the hardware, including peripherals such as a printer, a computer mouse or a keyboard. When you run an app, it sends 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). Similar to a personal computer, web servers also use an Operating System that functions as a host for all the software installed on them, which includes script applications or server-side software such as a flash server, a VOIP server, etc. A virtual machine can be generated on a physical one, so you can install a guest OS on it and have a different system environment compared to the physical server host Operating System.