An Operating System is the low-level software which manages a computer. It controls how the hardware behaves, the communication with peripherals for instance a monitor, a printer or a keyboard, the allocation of memory between various applications, and the prioritization of apps if several run concurrently. Each program that is installed on a PC works by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS is possible using a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what controls a web hosting server along with 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. When there are virtual machines created, they use a guest OS which runs on the server host OS.