An Operating System is the software which communicates with the hardware of a computer and functions as the intermediary between all the applications and the hardware, including peripherals such as a printer, a mouse or a keyboard. When you run software, it sends requests to the OS using an API (Application Program Interface) and the communication is done through a command line or a Graphical User Interface (GUI). Just like a personal computer, web servers also feature an Operating System that serves as a host for the software installed on them, including script apps or server-side software such as a flash server, a VOIP server, etc. A virtual machine can be set up on a physical one, so it is possible to install a guest OS on it and have a different system environment in contrast to the physical server host OS.