An Operating System is the fundamental software which manages a computer. It controls how your hardware behaves, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between the installed applications, along with the prioritization of applications in case multiple ones run simultaneously. Any program which is installed on a desktop or a laptop works by sending requests to the OS for different services using an application program interface (API). The communication with the OS can be performed through a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what controls a web server and all of the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. In case there're virtual machines created, they employ a guest OS which runs on the server host OS.