An Operating System is the software which interacts with the hardware of a computer and serves as the intermediary between applications and the hardware, including peripheral devices 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). A lot like a personal computer, web servers also have an Operating System that functions as a host for all the software installed on them, including script apps or server-side software such as a flash server, a VOIP server, etc. 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.