An Operating System is the basic software that manages a computer system. It handles the way your hardware functions, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications when multiple ones run at once. Any 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 communication with the OS is possible through a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what controls a web server as well as all of the 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 work with a guest OS that runs on the server host OS.