An Operating System is the fundamental software that manages a computer system. It handles how the hardware behaves, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between apps, plus the prioritization of apps in case several run at the same time. Each program that's installed on a PC operates by sending requests to the OS for various services via 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 web hosting world, the Operating System is what controls a hosting server along with all the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. When there are virtual machines created, they use a guest OS which runs on the server host OS.