An Operating System is the fundamental software which controls a computer. It handles the way your hardware behaves, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications if multiple ones run at the same time. Each program that is installed on a computer operates by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what commands a hosting server as well as all the software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. If there're virtual machines created, they employ a guest OS that runs on the server host OS.