An Operating System is the fundamental software which manages a PC. It handles the way your hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between your applications, along with the prioritization of apps in case several run at once. Any program which is installed on a PC functions by sending requests to the OS for various services via an application program interface (API). The communication with the OS can be achieved through a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the OS is what commands a web server as well as any software installed on it, which includes not only website scripts, but also any other application for example a VOIP or a game server. When there're virtual machines created, they work with a guest OS which is run on the server host OS.