An Operating System is the low-level software which manages a computer system. It handles how the hardware functions, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications when several run concurrently. Each and every program that is installed on a computer system functions by sending requests to the OS for different services via an application program interface (API). The interaction with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the OS is what runs a server and all 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. If there are virtual machines created, they work with a guest OS that is run on the server host OS.