An Operating System is the low-level software that manages a computer. It handles how the hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between the installed applications, plus the prioritization of apps in case several run simultaneously. 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 using a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what runs a server as well as any 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 which runs on the server host OS.