An Operating System is the low-level software that manages a PC. It handles the way the hardware behaves, the communication with peripherals such as a monitor, a printer or a keyboard, the allocation of memory between the installed applications, along with the prioritization of apps if several run simultaneously. Every program that is installed on a computer system operates by sending requests to the OS for various services using an application program interface (API). The interaction with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the Internet hosting field, the OS is what controls a server as well as any software installed on it, which includes not only website scripts, but also any other application such as a VOIP or a game server. In case there are virtual machines created, they work with a guest OS that runs on the server host OS.