An Operating System is the software which communicates with the hardware of a PC and is the intermediary between apps and the hardware, which includes peripheral devices such as a printer, a mouse or a keyboard. When you run an app, it sends requests to the OS using an API (Application Program Interface) and the communication is done through a command line or a Graphical User Interface (GUI). Just like a personal computer, web servers also feature an Operating System that acts as a host for any software installed on them, including script applications or server-side software such as a flash server, a VOIP server, etcetera. A virtual machine can be generated on a physical one, so you will be able to install a guest OS on it and have a different system environment in contrast to the physical server host Operating System.