An Operating System is the software that interacts with the hardware of a computer and functions as the intermediary between all the applications and the hardware, including peripheral devices such as a printer, a mouse or a keyboard. When you run software, it transmits requests to the OS through 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 servers also feature an Operating System that serves 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 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.