An Operating System is the software that interacts with the hardware of a computer system and serves as the intermediary between apps and the hardware, which includes peripheral devices such as a printer, a computer mouse or a keyboard. When you run an application, it transmits requests to the OS using an API (Application Program Interface) and the interaction is done through 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 any software installed on them, including script applications or server-side software such as a flash server, a VOIP server, and many others. 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 compared to the physical server host OS.