An Operating System is the fundamental software that manages a computer system. It handles the way your hardware behaves, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of applications when multiple ones run at once. Every program that's installed on a desktop or a notebook operates by sending requests to the OS for different services using an application program interface (API). The interaction with the OS is possible via a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what commands a hosting server and all the software installed on it, which includes not only website scripts, but also any other application such as a VOIP or a game server. If there're virtual machines created, they use a guest OS which is run on the server host OS.