An Operating System is the software that interacts with the hardware of a computer system and functions as the intermediary between 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 using 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, hosting servers also have 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, etc. A virtual machine can be created on a physical one, so you can install a guest OS on it and have a different system environment in contrast with the physical server host OS.