An Operating System is the fundamental software that controls a PC. It takes care of how the hardware behaves, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications when several run simultaneously. Every program that is installed on a desktop or a laptop functions by sending requests to the OS for different services via an application program interface (API). The contact with the OS is possible using a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a web hosting server along with any software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. If there're virtual machines created, they employ a guest OS that is run on the server host OS.