An Operating System is the fundamental software which manages a PC. It controls the way the hardware operates, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of apps in case several run simultaneously. Each and every program that is installed on a desktop or a notebook operates 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 web hosting field, the OS is what controls a server together with any software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. If there're virtual machines created, they work with a guest OS that is run on the server host OS.