An Operating System is the fundamental software that controls a computer. It controls how your hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between your applications, plus the prioritization of apps in case multiple ones run concurrently. Any program that is installed on a computer system operates by sending requests to the OS for different services using an application program interface (API). The interaction with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the Operating System is what commands a web server and all of the software installed on it, which includes not only website scripts, but also every other application such as a VOIP or a game server. When there are virtual machines created, they work with a guest OS which runs on the server host OS.