An Operating System is the low-level software that controls a computer. It controls the way your hardware behaves, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between all the applications, as well as the prioritization of applications if several run concurrently. Each program which is installed on a desktop or a notebook operates by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be done via a Graphical User Interface (GUI) or a command line. In the web hosting world, the OS is what controls a web server as well as any software installed on it, which includes not only website scripts, but also every other application such as a VOIP or a game server. If there are virtual machines created, they work with a guest OS that is run on the server host OS.