An Operating System is the low-level software that controls a computer system. It takes care of the way the hardware behaves, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between all the applications, as well as the prioritization of apps in case multiple ones run at the same time. Each and every program which is installed on a notebook or a desktop works by sending requests to the OS for different services via an application program interface (API). The communication with the OS can be done via a Graphical User Interface (GUI) or a command line. In the web hosting world, the OS is what runs a web server as well as any software installed on it, which includes not only website scripts, but, in addition every other application for example 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.