An Operating System is the low-level software which controls a PC. It handles the way the hardware behaves, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between the installed applications, and the prioritization of applications when multiple ones run simultaneously. Each and every program which is installed on a desktop or a laptop operates by sending requests to the OS for various services using an application program interface (API). The communication with the OS can be done using a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what commands a web hosting server together with all of the software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. When there are virtual machines created, they use a guest OS that runs on the server host OS.