An Operating System is the low-level software that manages a PC. It controls how your hardware behaves, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between your applications, along with the prioritization of applications if several run at the same time. Every program that is installed on a PC functions by sending requests to the OS for various services via an application program interface (API). The interaction with the OS can be done via a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the OS is what runs a hosting server and all the software installed on it, which includes not only website scripts, but, in addition every other application for instance a VOIP or a game server. When there are virtual machines created, they employ a guest OS which runs on the server host OS.