An Operating System is the basic software that controls a computer. It controls the way the hardware behaves, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between the installed 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 notebook works by sending requests to the OS for various services by using an application program interface (API). The communication with the OS can be achieved through a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what commands a web server and all of the software installed on it, which includes not only website scripts, but also every other application such as a VOIP or a game server. If there're virtual machines created, they use a guest OS which is run on the server host OS.