An Operating System is the fundamental software that controls a computer system. It takes care of how your hardware behaves, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between apps, plus the prioritization of apps in case several run at once. Any program that's installed on a notebook or a desktop works by sending requests to the OS for different services by using an application program interface (API). The contact with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what runs a server as well as any software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. If there are virtual machines created, they use a guest OS that is run on the server host OS.