An Operating System is the low-level software that 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, along with the prioritization of applications when several run simultaneously. Each and every program that is installed on a notebook or a desktop operates by sending requests to the OS for different services by using an application program interface (API). The communication with the OS is possible via a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what commands a web hosting server as well as all the software installed on it, which includes not only website scripts, but also any other application such as 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.