An Operating System is the low-level software that controls a computer. It takes care of the way your hardware operates, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of applications when several run simultaneously. Each program that's installed on a PC operates by sending requests to the OS for different services via an application program interface (API). The contact with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the hosting field, the Operating System is what commands a web hosting server along with 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're virtual machines created, they work with a guest OS that is run on the server host OS.