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