An Operating System is the fundamental software that controls a computer system. It takes care of the way the hardware operates, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of applications when several run at the same time. Any program that's installed on a computer system operates by sending requests to the OS for various services via an application program interface (API). The communication with the OS can be done via a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what controls a hosting server and any software installed on it, which includes not only website scripts, but, in addition any other application for example 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.