An Operating System is the basic software which controls a computer. It takes care of how your hardware operates, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between various applications, as well as the prioritization of applications when multiple ones run at the same time. Every program that's installed on a laptop or a desktop operates by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what runs a web hosting server as well as all the software installed on it, which includes not only website scripts, but, in addition every other application for instance a VOIP or a game server. When there are virtual machines created, they employ a guest OS that is run on the server host OS.