An Operating System is the basic software which manages a PC. It takes care of how your hardware operates, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between applications, plus the prioritization of applications in case several run simultaneously. Each and every program that's installed on a computer works by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what controls a web hosting server as well as all of the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. When there are virtual machines created, they employ a guest OS that runs on the server host OS.