An Operating System is the basic software that controls a computer system. It controls how the hardware functions, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between the installed applications, and the prioritization of applications when multiple ones run at the same time. Each and every program that's installed on a desktop or a laptop operates by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what controls a hosting server together with all the software installed on it, which includes not only website scripts, but also any other application for instance a VOIP or a game server. If there're virtual machines created, they work with a guest OS which is run on the server host OS.