An Operating System is the low-level software that manages a computer system. 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, along with the prioritization of applications in case several run concurrently. Any program that's installed on a computer works by sending requests to the OS for various services via an application program interface (API). The interaction with the OS can be done through a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what commands a web server and all of the software installed on it, which includes not only website scripts, but also any other application for example a VOIP or a game server. In case there're virtual machines created, they work with a guest OS which is run on the server host OS.