An Operating System is the basic software which manages a computer. It takes care of the way the hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between your applications, plus the prioritization of apps if multiple ones run at the same time. Each program which is installed on a computer operates by sending requests to the OS for various services by using an application program interface (API). The contact with the OS can be done using a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a server along with 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. When there are virtual machines created, they work with a guest OS which runs on the server host OS.