An Operating System is the basic software which manages a computer system. It handles how the hardware functions, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between the installed applications, and the prioritization of apps when multiple ones run at once. Each program that's installed on a PC works by sending requests to the OS for various services by using an application program interface (API). The interaction with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what runs a web server as well as any software installed on it, which includes not only website scripts, but, in addition every other application for example a VOIP or a game server. When there are virtual machines created, they use a guest OS that is run on the server host OS.