An Operating System is the software which interacts with the hardware of a PC and functions as the intermediary between all the applications and the hardware, which includes peripheral devices such as a printer, a computer mouse or a keyboard. When you run an app, it sends requests to the OS through an API (Application Program Interface) and the communication 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 serves as a host for all of the software installed on them, which includes script apps or server-side software such as a flash server, a VOIP server, etcetera. A virtual machine can be generated on a physical one, so you can install a guest OS on it and have a different system environment in contrast with the physical server host OS.