An Operating System is the basic software which controls a computer. It handles the way your hardware behaves, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between apps, along with the prioritization of applications in case multiple ones run at once. Every program that is installed on a laptop or a desktop operates by sending requests to the OS for different services by using an application program interface (API). The communication with the OS is possible via a Graphical User Interface (GUI) or a command line. In the 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 such as a VOIP or a game server. In case there're virtual machines created, they work with a guest OS which runs on the server host OS.