An Operating System is the fundamental software which controls a PC. It handles how your hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between applications, along with the prioritization of applications in case several run concurrently. Each and every program that is installed on a laptop or a desktop functions by sending requests to the OS for various services by using an application program interface (API). The interaction with the OS can be done using a Graphical User Interface (GUI) or a command line. In the hosting world, the OS 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 every other application such as a VOIP or a game server. In case there're virtual machines created, they use a guest OS which is run on the server host OS.