An Operating System is the fundamental software that controls a computer. 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 various applications, and the prioritization of apps in case several run simultaneously. Each and every program that is installed on a desktop or a notebook operates by sending requests to the OS for various services using an application program interface (API). The communication with the OS is possible using a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the Operating System is what runs a web hosting server and all the software installed on it, which includes not only website scripts, but, in addition every other application such as a VOIP or a game server. In case there're virtual machines created, they work with a guest OS which is run on the server host OS.