An Operating System is the basic software which controls a PC. It controls how your 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 apps when several run concurrently. Every program that is installed on a computer system works by sending requests to the OS for various 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 web hosting server along with all of the software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. In case there're virtual machines created, they employ a guest OS which runs on the server host OS.