An Operating System is the low-level software that controls a computer. It handles the way your hardware behaves, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between applications, plus the prioritization of applications when multiple ones run simultaneously. Each program that is installed on a laptop or a desktop operates by sending requests to the OS for different services using an application program interface (API). The communication with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what runs a web server as well as all of the software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. When there are virtual machines created, they employ a guest OS which runs on the server host OS.