An Operating System is the basic software which controls a PC. It takes care of the way your hardware behaves, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between your applications, along with the prioritization of apps if several run concurrently. Each and every program that's installed on a computer functions by sending requests to the OS for various services via 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 OS is what commands a web server and all the software installed on it, which includes not only website scripts, but, in addition every other application such as a VOIP or a game server. In case there're virtual machines created, they employ a guest OS that is run on the server host OS.