An Operating System is the low-level software which controls a computer system. It handles how the hardware functions, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between the installed applications, and the prioritization of apps if multiple ones run concurrently. Each program that's installed on a notebook or a desktop works by sending requests to the OS for different services using an application program interface (API). The communication with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a server along with any software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. If there're virtual machines created, they use a guest OS that runs on the server host OS.