An Operating System is the fundamental software which controls a computer. It takes care of how the hardware operates, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of applications if several run at the same time. Each and every program that's installed on a desktop or a laptop functions by sending requests to the OS for different services via an application program interface (API). The interaction with the OS is possible through a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a web hosting server along with any software installed on it, which includes not only website scripts, but, in addition every other application for instance a VOIP or a game server. If there're virtual machines created, they work with a guest OS which runs on the server host OS.