An Operating System is the software that communicates with the hardware of a computer and works as the intermediary between apps and the hardware, which includes peripheral devices such as a printer, a computer mouse or a keyboard. When you run an application, 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). Identical to a personal computer, web servers also use an Operating System that acts as a host for the software installed on them, including script apps or server-side software such as a flash server, a VOIP server, and many others. A virtual machine can be created on a physical one, so you'll be able to install a guest OS on it and have a different system environment in contrast to the physical server host Operating System.