An Operating System is the basic software which manages a computer. It takes care of how your hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of applications in case multiple ones run at the same time. Every program that is installed on a notebook or a desktop works by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS is possible through a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what controls a server together with any software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. If there are virtual machines created, they use a guest OS that runs on the server host OS.