An Operating System is the basic software that controls a PC. It takes care of how your hardware functions, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of apps in case multiple ones run at the same time. Each and every program that's installed on a desktop or a laptop functions by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what commands a web hosting 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. When there're virtual machines created, they employ a guest OS that runs on the server host OS.