An Operating System is the fundamental software which controls a computer. It handles how the hardware functions, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between various applications, plus the prioritization of applications when several run at once. Every program which is installed on a desktop or a laptop works by sending requests to the OS for different services using an application program interface (API). The interaction with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the Operating System is what commands a hosting server along with all the software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. If there're virtual machines created, they employ a guest OS which is run on the server host OS.