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