An Operating System is the software which interacts with the hardware of a PC and serves as the intermediary between all the applications and the hardware, including peripheral devices such as a printer, a computer mouse or a keyboard. When you run software, it transmits requests to the OS through an API (Application Program Interface) and the contact is done through a command line or a Graphical User Interface (GUI). Just like a personal computer, web hosting servers also use an Operating System that serves 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, etcetera. 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 with the physical server host Operating System.