An Operating System is the low-level software that controls a PC. It controls how your hardware behaves, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications in case multiple ones run at once. Every program that is installed on a desktop or a notebook works by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be done through a Graphical User Interface (GUI) or a command line. In the web hosting world, the OS is what commands a 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. If there're virtual machines created, they work with a guest OS that runs on the server host OS.