An Operating System is the low-level software that controls a PC. It takes care of how your hardware behaves, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between all the applications, plus the prioritization of apps when multiple ones run at once. Every program that's installed on a PC operates by sending requests to the OS for various services by using an application program interface (API). The contact with the OS can be done via a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what runs a web hosting server along with all the software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. In case there're virtual machines created, they work with a guest OS that runs on the server host OS.