An Operating System is the basic software which manages a computer. It controls how the hardware behaves, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between applications, along with the prioritization of applications when several run at once. Every program that is installed on a desktop or a notebook functions by sending requests to the OS for various services by using an application program interface (API). The contact with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what commands a server together with any 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 runs on the server host OS.