An Operating System is the low-level software which manages a computer system. It takes care of the way your hardware operates, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications if several run simultaneously. Every program that is installed on a desktop or a laptop functions by sending requests to the OS for different services using an application program interface (API). The interaction 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 commands a web server together with all of the software installed on it, which includes not only website scripts, but also 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.