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