An Operating System is the basic software which controls a computer. It handles the way your hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between your applications, and the prioritization of apps in case several run at once. Every program which is installed on a laptop or a desktop operates by sending requests to the OS for various services by using an application program interface (API). The communication with the OS can be done through a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what controls a hosting server together with any software installed on it, which includes not only website scripts, but also every other application such as a VOIP or a game server. When there are virtual machines created, they use a guest OS that runs on the server host OS.