An Operating System is the fundamental software that manages a PC. It controls the way your hardware operates, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between various applications, plus the prioritization of apps in case several run simultaneously. Every program which is installed on a desktop or a notebook functions by sending requests to the OS for various services via an application program interface (API). The contact with the OS can be done using a Graphical User Interface (GUI) or a command line. In the web hosting world, the OS is what runs a hosting server and all of the software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. When there're virtual machines created, they work with a guest OS that is run on the server host OS.