An Operating System is the fundamental software that manages a PC. It handles the way your hardware behaves, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between the installed applications, plus the prioritization of apps in case multiple ones run at once. Any program that is installed on a computer works by sending requests to the OS for various services by using an application program interface (API). The interaction with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what commands a hosting server and any software installed on it, which includes not only website scripts, but also any other application for instance a VOIP or a game server. In case there're virtual machines created, they employ a guest OS which is run on the server host OS.