An Operating System is the basic software that controls a computer. It controls the way your hardware behaves, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of applications when several run concurrently. Each and every program which is installed on a computer functions by sending requests to the OS for various services via an application program interface (API). The contact with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what commands a web hosting server as well as any software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. When there are virtual machines created, they use a guest OS which is run on the server host OS.