An Operating System is the basic software which controls a computer system. It handles how your hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between your applications, as well as the prioritization of apps when multiple ones run simultaneously. Every program that is installed on a notebook or a desktop operates by sending requests to the OS for various services by using an application program interface (API). The interaction with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what runs a hosting server together with 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. In case there're virtual machines created, they employ a guest OS which is run on the server host OS.