An Operating System is the fundamental software which manages a computer system. It takes care of how your hardware operates, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between all the applications, plus the prioritization of applications in case multiple ones run at the same time. Each program that's installed on a desktop or a notebook works by sending requests to the OS for various services by using an application program interface (API). The contact with the OS is possible using a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what controls a web server along with any software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. In case there are virtual machines created, they use a guest OS that is run on the server host OS.