An Operating System is the low-level software which manages a PC. It controls the way your hardware functions, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between all the applications, as well as the prioritization of applications when several run concurrently. Any program that is installed on a notebook or a desktop works by sending requests to the OS for various services via an application program interface (API). The communication with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what commands a web server and all the software installed on it, which includes not only website scripts, but also any other application for example a VOIP or a game server. If there're virtual machines created, they employ a guest OS which is run on the server host OS.