An Operating System is the basic software which manages a computer. It takes care of how your hardware operates, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between various applications, plus the prioritization of applications in case multiple ones run at once. Any program that is installed on a notebook or a desktop functions by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what commands a server and any 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 work with a guest OS which is run on the server host OS.