An Operating System is the fundamental software that controls a computer. It controls the way your hardware behaves, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of apps if several run simultaneously. Each and every program which is installed on a PC works by sending requests to the OS for different services using an application program interface (API). The communication with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what controls a hosting server together with all of the software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. If there are virtual machines created, they employ a guest OS that runs on the server host OS.