An Operating System is the fundamental software which controls a computer. It controls how your hardware behaves, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications if multiple ones run simultaneously. Each and every program that is installed on a laptop or a desktop functions by sending requests to the OS for different services via 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 hosting field, the OS is what runs a web hosting server along with all the software installed on it, which includes not only website scripts, but, in addition every other application such as a VOIP or a game server. If there're virtual machines created, they use a guest OS that is run on the server host OS.