An Operating System is the low-level software which manages a computer system. It controls how your hardware functions, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between applications, and the prioritization of applications when multiple ones run simultaneously. Each and every program which is installed on a desktop or a notebook operates by sending requests to the OS for different services using an application program interface (API). The contact with the OS is possible via a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what runs a hosting server along with all of the software installed on it, which includes not only website scripts, but, in addition every other application for example a VOIP or a game server. When there're virtual machines created, they employ a guest OS that is run on the server host OS.