An Operating System is the basic software that controls a computer system. It takes care of the way the hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of applications if multiple ones run concurrently. Every program that is installed on a notebook or a desktop works by sending requests to the OS for various services using an application program interface (API). The interaction with the OS can be done through a Graphical User Interface (GUI) or a command line. In the hosting field, the Operating System is what commands a server together with all of the software installed on it, which includes not only website scripts, but also any other application such as a VOIP or a game server. In case there are virtual machines created, they work with a guest OS that is run on the server host OS.