An Operating System is the basic software which manages a PC. It takes care of how your hardware functions, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between your applications, and the prioritization of apps if multiple ones run at the same time. Every program which is installed on a laptop or a desktop operates by sending requests to the OS for various services via an application program interface (API). The interaction with the OS is possible through a Graphical User Interface (GUI) or a command line. In the hosting field, the Operating System is what commands a hosting server and all of the software installed on it, which includes not only website scripts, but, in addition every other application for example a VOIP or a game server. If there're virtual machines created, they work with a guest OS that is run on the server host OS.