An Operating System is the fundamental software that controls a computer. It handles how your hardware operates, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of apps when several run at once. Each program that's installed on a computer operates by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what commands a server as well as all the software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. In case there are virtual machines created, they employ a guest OS which runs on the server host OS.