An Operating System is the low-level software that manages a computer system. It takes care of how your hardware behaves, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between apps, and the prioritization of apps if several run at the same time. Each and every program which is installed on a PC functions by sending requests to the OS for various services via an application program interface (API). The interaction with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what controls a hosting server together with all the software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. When there're virtual machines created, they employ a guest OS which runs on the server host OS.