An Operating System is the low-level software which manages a computer. It handles how the hardware functions, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between apps, plus the prioritization of apps if several run at once. Each program which is installed on a computer system works by sending requests to the OS for different services by using an application program interface (API). The contact with the OS can be done using a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what controls a web hosting server as well as any software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. When there're virtual machines created, they work with a guest OS which runs on the server host OS.