An Operating System is the low-level software which controls a PC. It controls the way the hardware functions, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between the installed applications, as well as the prioritization of applications in case several run at the same time. Every program which is installed on a computer works by sending requests to the OS for various services via an application program interface (API). The communication with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what commands a web hosting server and 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 use a guest OS which is run on the server host OS.