An Operating System is the software that interacts with the hardware of a computer and serves as the intermediary between all the applications 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 contact is done through a command line or a Graphical User Interface (GUI). Identical to a personal computer, web hosting servers also have an Operating System that acts as a host for any software installed on them, which includes script applications or server-side software such as a flash server, a VOIP server, etc. A virtual machine can be set up on a physical one, so you can install a guest OS on it and have a different system environment compared to the physical server host OS.