An Operating System is the fundamental software that controls a PC. It handles how your hardware behaves, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between your applications, and the prioritization of applications if several run concurrently. Each and every program that is installed on a desktop or a notebook works by sending requests to the OS for different services via an application program interface (API). The interaction with the OS is possible using a Graphical User Interface (GUI) or a command line. In the hosting field, the Operating System is what controls a web server along with all the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. When there're virtual machines created, they work with a guest OS which runs on the server host OS.