An Operating System is the low-level software which controls a computer. It handles how your hardware operates, the communication with peripheral devices for instance a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications when multiple ones run simultaneously. Each and every program that 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 performed through a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a web hosting server together with any software installed on it, which includes not only website scripts, but also 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.