An Operating System is the low-level software that manages a computer system. It takes care of how the hardware operates, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between the installed applications, as well as the prioritization of apps in case multiple ones run at the same time. Every program which is installed on a computer system operates by sending requests to the OS for different services by using an application program interface (API). The contact with the OS can be done via a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what controls a web hosting server and all the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. If there're virtual machines created, they work with a guest OS that runs on the server host OS.