An Operating System is the basic software that manages a PC. It controls how the hardware behaves, the communication with peripherals including a monitor, a printer or a keyboard, the allocation of memory between apps, plus the prioritization of apps if multiple ones run at once. Each and every program which is installed on a computer operates 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 hosting world, the Operating System is what commands a hosting server and all of the software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. In case there are virtual machines created, they work with a guest OS which runs on the server host OS.