An Operating System is the fundamental software that controls a PC. It handles the way the hardware operates, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between your applications, as well as the prioritization of apps in case several run simultaneously. Each and every program that is installed on a notebook or a desktop operates by sending requests to the OS for different services using an application program interface (API). The contact with the OS is possible through a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what controls a web hosting server as well as any software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. If there are virtual machines created, they employ a guest OS which runs on the server host OS.