An Operating System is the low-level software that manages a computer. It takes care of the way the hardware functions, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of applications in case multiple ones run simultaneously. 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 communication with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what commands a web server and all of the software installed on it, which includes not only website scripts, but also any other application for example a VOIP or a game server. When there are virtual machines created, they work with a guest OS that runs on the server host OS.