An Operating System is the basic software that manages a PC. It controls how your hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between the installed applications, and the prioritization of applications if several run concurrently. Each and every program that's installed on a PC functions by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be done using a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what commands a web server together with any software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. When there're virtual machines created, they work with a guest OS that runs on the server host OS.