An Operating System is the software which communicates with the hardware of a PC and is the intermediary between the installed applications and the hardware, which includes peripherals such as a printer, a mouse or a keyboard. When you run software, it sends requests to the OS through 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 hosting servers also feature an Operating System that 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, etc. A virtual machine can be set up on a physical one, so you can install a guest OS on it and have a different system environment compared to the physical server host Operating System.