An Operating System is the low-level software which controls a PC. It takes care of the way your hardware functions, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between apps, as well as the prioritization of applications when several run at once. Each and every program that's installed on a laptop or a desktop operates by sending requests to the OS for different services using an application program interface (API). The interaction with the OS is possible using a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what controls a hosting server together with all of the software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. In case there are virtual machines created, they use a guest OS that runs on the server host OS.