An Operating System is the basic software that controls a computer. It controls how your hardware operates, the communication with peripherals like a monitor, a printer or a keyboard, the allocation of memory between various applications, plus the prioritization of apps if several run at the same time. Every program that's installed on a PC functions by sending requests to the OS for various services using an application program interface (API). The communication with the OS is possible through a Graphical User Interface (GUI) or a command line. In the web hosting field, the Operating System is what controls a hosting server and all the software installed on it, which includes not only website scripts, but also every other application for instance a VOIP or a game server. In case there are virtual machines created, they work with a guest OS which is run on the server host OS.