An Operating System is the low-level software which controls a PC. It controls how the hardware behaves, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between all the applications, and the prioritization of applications in case several run simultaneously. Each program that's installed on a desktop or a laptop operates by sending requests to the OS for different services using an application program interface (API). The communication 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 hosting server together with all the software installed on it, which includes not only website scripts, but also every other application for example a VOIP or a game server. When there are virtual machines created, they work with a guest OS which is run on the server host OS.