An Operating System is the software that interacts with the hardware of a computer system and is the intermediary between apps and the hardware, which includes peripherals such as a printer, a 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 through a command line or a Graphical User Interface (GUI). Similar to a personal computer, hosting servers also feature an Operating System which acts as a host for the software installed on them, which includes script apps or server-side software such as a flash server, a VOIP server, and so on. A virtual machine can be created on a physical one, so you could install a guest OS on it and have a different system environment in contrast to the physical server host OS.