An Operating System is the software that interacts with the hardware of a computer system and is 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 contact is done through a command line or a Graphical User Interface (GUI). Just like a personal computer, hosting servers also use an Operating System which acts as a host for all 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 set up on a physical one, so it is possible to install a guest OS on it and have a different system environment compared to the physical server host OS.