An Operating System is the fundamental software that controls a PC. It handles the way the hardware functions, 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 multiple ones run concurrently. Any program that's installed on a computer system functions by sending requests to the OS for various services using an application program interface (API). The communication with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what commands a server as well as any software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. In case there're virtual machines created, they use a guest OS that runs on the server host OS.