An Operating System is the low-level software which controls a computer. It controls how your hardware functions, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of applications in case several run at once. Each program which is installed on a PC operates by sending requests to the OS for different services by using an application program interface (API). The contact with the OS is possible using a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the Operating System is what commands a web server and any software installed on it, which includes not only website scripts, but also any other application for instance a VOIP or a game server. When there are virtual machines created, they use a guest OS that runs on the server host OS.