An Operating System is the fundamental software which manages a computer. It takes care of how the hardware behaves, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between your applications, plus the prioritization of applications when several run at once. Every program which is installed on a computer system functions by sending requests to the OS for various services using an application program interface (API). The interaction with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the web hosting world, the OS is what controls a web server along with all the software installed on it, which includes not only website scripts, but also any other application for instance a VOIP or a game server. When there're virtual machines created, they work with a guest OS that is run on the server host OS.