An Operating System is the software which communicates with the hardware of a computer system and serves as the intermediary between applications and the hardware, including peripherals such as a printer, a computer mouse or a keyboard. When you run an application, it transfers 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). Just like a personal computer, hosting servers also use an Operating System which acts 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 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 Operating System.