An Operating System is the basic software which manages a computer. It takes care of how your hardware operates, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between various applications, as well as the prioritization of applications in case multiple ones run concurrently. Every program that's installed on a desktop or a laptop operates by sending requests to the OS for different services via an application program interface (API). The interaction with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what runs a server and all the software installed on it, which includes not only website scripts, but also any other application for instance a VOIP or a game server. In case there are virtual machines created, they employ a guest OS that is run on the server host OS.