An Operating System is the low-level software that manages a computer system. It takes care of how the hardware functions, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between applications, as well as the prioritization of applications when multiple ones run simultaneously. Every program which is installed on a desktop or a laptop operates by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what runs a server together with all of the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. In case there are virtual machines created, they work with a guest OS which runs on the server host OS.