An Operating System is the basic software that manages a computer system. It handles how your hardware operates, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between all the applications, plus the prioritization of applications in case several run at once. Any program which is installed on a notebook or a desktop operates by sending requests to the OS for various services by using an application program interface (API). The communication with the OS can be done through a Graphical User Interface (GUI) or a command line. In the hosting field, the Operating System is what runs a server as well as all of the software installed on it, which includes not only website scripts, but, in addition every other application for instance a VOIP or a game server. In case there are virtual machines created, they work with a guest OS that runs on the server host OS.