An Operating System is the basic software which manages a computer system. It takes care of how the hardware behaves, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between apps, plus the prioritization of applications if multiple ones run simultaneously. Every program which is installed on a desktop or a laptop operates by sending requests to the OS for different services using an application program interface (API). The contact with the OS can be achieved using a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what runs a server as well as all of the software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. If there are virtual machines created, they employ a guest OS which is run on the server host OS.