An Operating System is the fundamental software that controls a computer. It takes care of how the hardware operates, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between applications, along with the prioritization of applications if multiple ones run at the same time. Each 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 is possible through a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what controls a web hosting server as well as any software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. In case there are virtual machines created, they work with a guest OS which runs on the server host OS.