An Operating System is the fundamental software which manages a computer system. It takes care of the way your hardware functions, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between the installed applications, and the prioritization of apps when several run concurrently. Each program that is installed on a computer operates by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what commands a web hosting server and any software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. When there're virtual machines created, they use a guest OS which is run on the server host OS.