An Operating System is the low-level software that controls a PC. It takes care of how your hardware behaves, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of applications when multiple ones run at once. Each program that is installed on a PC works by sending requests to the OS for various services via an application program interface (API). The interaction with the OS is possible via a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what runs a web hosting server and any 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 are virtual machines created, they employ a guest OS that is run on the server host OS.