An Operating System is the basic software that controls a PC. It controls how the hardware behaves, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between apps, plus the prioritization of applications when multiple ones run at the same time. Any program that's installed on a desktop or a laptop works by sending requests to the OS for various services by using an application program interface (API). The contact with the OS can be done using a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what commands a server along with all of the software installed on it, which includes not only website scripts, but, in addition any other application for example a VOIP or a game server. In case there're virtual machines created, they employ a guest OS that is run on the server host OS.