An Operating System is the basic software which controls a PC. It controls how your hardware behaves, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between your applications, as well as the prioritization of apps in case multiple ones run at once. Each and every program that is installed on a PC functions by sending requests to the OS for different services via an application program interface (API). The communication with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what controls a server as well as any 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 work with a guest OS which runs on the server host OS.