An Operating System is the fundamental software that manages a computer system. It handles how your hardware functions, the communication with peripherals 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 program that is installed on a computer works by sending requests to the OS for various services by using an application program interface (API). The interaction with the OS can be done via a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what runs a server together with all the software installed on it, which includes not only website scripts, but also any other application such as a VOIP or a game server. When there are virtual machines created, they employ a guest OS that is run on the server host OS.