An Operating System is the low-level software that controls a PC. It controls how the hardware operates, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between your applications, as well as the prioritization of apps when multiple ones run at the same time. Every program that is installed on a computer operates by sending requests to the OS for different services via an application program interface (API). The interaction with the OS is possible via a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the OS is what commands a web server as well as 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. If there're virtual machines created, they work with a guest OS that runs on the server host OS.