An Operating System is the basic software that controls a PC. It handles the way the hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between your applications, plus the prioritization of applications if several run simultaneously. Each program that's installed on a notebook or a desktop operates by sending requests to the OS for various services via an application program interface (API). The interaction with the OS can be achieved through a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what runs a server along with any software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. When there are virtual machines created, they work with a guest OS which runs on the server host OS.