An Operating System is the low-level software that manages a PC. It takes care of how the hardware behaves, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of apps if multiple ones run concurrently. Every program that's installed on a computer system functions by sending requests to the OS for various services by using an application program interface (API). The contact with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what runs a web 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. If there're virtual machines created, they work with a guest OS which runs on the server host OS.