An Operating System is the fundamental software which manages a PC. It takes care of how your hardware behaves, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between the installed applications, plus the prioritization of apps when several run at the same time. Any program that is installed on a desktop or a laptop functions by sending requests to the OS for different services using an application program interface (API). The interaction with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the OS is what controls a web server together with all of the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. In case there are virtual machines created, they use a guest OS which is run on the server host OS.