An Operating System is the low-level software that controls a PC. It takes care of how your hardware operates, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of apps when several run at the same time. Each program that is installed on a computer system functions by sending requests to the OS for various services via an application program interface (API). The interaction with the OS is possible using a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what commands a web server along with all the software installed on it, which includes not only website scripts, but, in addition every other application for instance a VOIP or a game server. When there're virtual machines created, they work with a guest OS which runs on the server host OS.