An Operating System is the basic software which controls a PC. It controls how your hardware functions, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between various applications, as well as the prioritization of apps in case several run simultaneously. Any program which is installed on a computer system works by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what commands a 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. If there are virtual machines created, they employ a guest OS that runs on the server host OS.