An Operating System is the basic software that manages a PC. It handles the way the hardware behaves, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between all the applications, as well as the prioritization of apps in case several run at the same time. Each program that's installed on a PC works by sending requests to the OS for various services 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 Internet hosting field, the OS is what commands a web hosting server and 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. In case there are virtual machines created, they employ a guest OS which runs on the server host OS.