An Operating System is the basic software that controls a computer. It takes care of the way your hardware functions, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between applications, plus the prioritization of apps if multiple ones run at once. Each program that's installed on a laptop or a desktop operates by sending requests to the OS for different services using an application program interface (API). The communication with the OS is possible using a Graphical User Interface (GUI) or a command line. In the web hosting world, the OS is what controls a hosting server together 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 are virtual machines created, they employ a guest OS that is run on the server host OS.