An Operating System is the low-level software which controls a PC. It takes care of how the hardware operates, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between applications, plus the prioritization of apps if several run at once. Each and every program that's installed on a computer operates by sending requests to the OS for different services via an application program interface (API). The contact with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what runs a server as well as all the software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. If there're virtual machines created, they employ a guest OS which runs on the server host OS.
