An Operating System is the basic software that manages a PC. It takes care of how the hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications when multiple ones run concurrently. Each and every program which is installed on a computer operates by sending requests to the OS for different services via an application program interface (API). The contact with the OS is possible through a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what runs a web hosting server together with any software installed on it, which includes not only website scripts, but also any other application such as a VOIP or a game server. When there are virtual machines created, they work with a guest OS which is run on the server host OS.