An Operating System is the fundamental software that manages a PC. It controls how your hardware operates, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between the installed applications, along with the prioritization of applications if multiple ones run concurrently. Every program that's installed on a PC functions by sending requests to the OS for different services via an application program interface (API). The interaction with the OS can be done using a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the Operating System is what commands a web server as well as any software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. If there are virtual machines created, they employ a guest OS which runs on the server host OS.