An Operating System is the low-level software that manages a computer. It handles the way the hardware operates, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications if several run concurrently. Each and every program that is installed on a notebook or a desktop functions by sending requests to the OS for various services using an application program interface (API). The communication with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what controls a server and all the software installed on it, which includes not only website scripts, but, in addition every other application for example a VOIP or a game server. If there're virtual machines created, they employ a guest OS which is run on the server host OS.