An Operating System is the software which interacts with the hardware of a PC and functions as the intermediary between apps and the hardware, which includes peripherals such as a printer, a mouse or a keyboard. When you run an application, it sends requests to the OS using an API (Application Program Interface) and the communication is done via a command line or a Graphical User Interface (GUI). A lot like a personal computer, web hosting servers also feature an Operating System that acts as a host for all of the software installed on them, which includes script apps or server-side software such as a flash server, a VOIP server, and many others. A virtual machine can be created 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 OS.