An Operating System is the software that interacts with the hardware of a computer system and serves as the intermediary between all the applications and the hardware, which includes peripherals such as a printer, a computer mouse or a keyboard. When you run an application, it transmits 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 feature an Operating System that functions as a host for all the software installed on them, including script applications or server-side software such as a flash server, a VOIP server, etc. 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 with the physical server host Operating System.
