An Operating System is the fundamental software that controls a PC. It takes care of the way the hardware functions, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between applications, as well as the prioritization of applications when multiple ones run at once. Any program that is installed on a laptop or a desktop operates by sending requests to the OS for various services via an application program interface (API). The communication with the OS can be achieved through a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what runs a hosting server as well as all the software installed on it, which includes not only website scripts, but, in addition every other application for example a VOIP or a game server. In case there're virtual machines created, they employ a guest OS which runs on the server host OS.