An Operating System is the low-level software that controls a computer system. It takes care of how the hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications if multiple ones run concurrently. Each and every program which is installed on a computer system functions by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be done via a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what commands a web hosting server and all the software installed on it, which includes not only website scripts, but also every other application for instance 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.