An Operating System is the basic software which controls a PC. It handles the way your hardware behaves, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between all the applications, plus the prioritization of applications when multiple ones run at the same time. Each program which is installed on a notebook or a desktop operates by sending requests to the OS for different 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 Internet hosting world, the OS is what controls a web server and any software installed on it, which includes not only website scripts, but, in addition 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.