An Operating System is the basic software which controls a computer. It takes care of how your hardware functions, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between your applications, and the prioritization of apps when multiple ones run at once. Any program that is installed on a laptop or a desktop works by sending requests to the OS for different services using an application program interface (API). The communication with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what runs a web hosting server together with any software installed on it, which includes not only website scripts, but, in addition every other application for instance a VOIP or a game server. When there are virtual machines created, they employ a guest OS that is run on the server host OS.