An Operating System is the low-level software which controls a computer. It takes care of how your hardware behaves, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between apps, and the prioritization of apps when multiple ones run concurrently. Any program which is installed on a desktop or a notebook works by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS is possible using a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what controls a web server along with all of the software installed on it, which includes not only website scripts, but also every other application such as a VOIP or a game server. When there are virtual machines created, they employ a guest OS that runs on the server host OS.