An Operating System is the software that communicates with the hardware of a PC and functions 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 transmits requests to the OS through an API (Application Program Interface) and the contact is done through a command line or a Graphical User Interface (GUI). Similar to a personal computer, web servers also feature an Operating System that 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, etc. 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 in contrast with the physical server host Operating System.
