An Operating System is the software which communicates with the hardware of a computer system and serves as the intermediary between apps and the hardware, including peripheral devices 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 interaction is done via a command line or a Graphical User Interface (GUI). A lot like a personal computer, hosting servers also feature an Operating System that functions as a host for all the software installed on them, which includes script applications or server-side software such as a flash server, a VOIP server, and many others. A virtual machine can be generated on a physical one, so it is possible to install a guest OS on it and have a different system environment compared to the physical server host OS.