An Operating System is the fundamental software that controls a computer system. It takes care of how the hardware functions, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications in case multiple ones run simultaneously. Each program that is installed on a laptop or a desktop works by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what commands a web hosting server along with all the software installed on it, which includes not only website scripts, but also any other application for instance a VOIP or a game server. When there are virtual machines created, they employ a guest OS which is run on the server host OS.