An Operating System is the fundamental software that manages a PC. It handles the way the hardware functions, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between the installed applications, along with the prioritization of apps when several run concurrently. Every program that's installed on a notebook or a desktop works by sending requests to the OS for various services using an application program interface (API). The interaction with the OS can be achieved through a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what controls a web server along with all the software installed on it, which includes not only website scripts, but also any other application for example a VOIP or a game server. If there're virtual machines created, they work with a guest OS which runs on the server host OS.