An Operating System is the fundamental software which controls a computer. It handles the way your hardware functions, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications if multiple ones run concurrently. Each and every program which is installed on a laptop or a desktop operates by sending requests to the OS for various services using 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 server together with all the software installed on it, which includes not only website scripts, but, in addition every other application such as a VOIP or a game server. When there're virtual machines created, they use a guest OS which runs on the server host OS.