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