An Operating System is the software that interacts with the hardware of a computer system and serves as 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). Similar to a personal computer, web servers also have an Operating System that serves as a host for all of the software installed on them, including script applications or server-side software such as a flash server, a VOIP server, and so on. A virtual machine can be generated on a physical one, so you could install a guest OS on it and have a different system environment compared to the physical server host Operating System.