An Operating System is the fundamental software which controls a PC. It controls the way your hardware behaves, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between the installed applications, plus the prioritization of apps if multiple ones run concurrently. Each program that is installed on a notebook or a desktop operates by sending requests to the OS for various services by using an application program interface (API). The communication with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what controls a server together with all of the software installed on it, which includes not only website scripts, but, in addition every other application such as a VOIP or a game server. When there are virtual machines created, they work with a guest OS that runs on the server host OS.