An Operating System is the basic software that manages a PC. It controls 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 if multiple ones run at once. Every program that is installed on a computer operates by sending requests to the OS for different services via an application program interface (API). The interaction with the OS is possible via a Graphical User Interface (GUI) or a command line. In the hosting field, the OS is what controls a web server together with any software installed on it, which includes not only website scripts, but also every other application such as a VOIP or a game server. When there're virtual machines created, they work with a guest OS which runs on the server host OS.
