An Operating System is the basic software which manages a computer. It handles the way your hardware operates, the communication with peripheral devices like a monitor, a printer or a keyboard, the allocation of memory between applications, and the prioritization of apps when several run concurrently. Any program that's installed on a computer works by sending requests to the OS for different services using an application program interface (API). The communication with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a web hosting server together with all the software installed on it, which includes not only website scripts, but, in addition every 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.