An Operating System is the fundamental software that controls a computer. It controls the way your hardware functions, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between your applications, and the prioritization of apps when several run concurrently. Any program that is installed on a computer works by sending requests to the OS for various services by using an application program interface (API). The interaction with the OS can be done via a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what runs a hosting server as well as all of the software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. When there're virtual machines created, they use a guest OS which is run on the server host OS.