An Operating System is the low-level software that controls a PC. It takes care of how your hardware functions, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of apps when several run concurrently. Every program that is installed on a PC functions by sending requests to the OS for various services via an application program interface (API). The communication with the OS can be done via a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what runs a server as well as all of the 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're virtual machines created, they work with a guest OS which is run on the server host OS.