An Operating System is the low-level software that controls a PC. It handles the way the hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between all the applications, along with the prioritization of applications when multiple ones run simultaneously. Each program that's installed on a computer system functions by sending requests to the OS for various services using an application program interface (API). The contact with the OS can be done via a Graphical User Interface (GUI) or a command line. In the web hosting world, the Operating System is what commands a server together with any software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. When there are virtual machines created, they work with a guest OS which runs on the server host OS.