An Operating System is the basic software which controls a PC. It takes care of the way your hardware functions, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between all the applications, along with the prioritization of applications if multiple ones run concurrently. Any program that's installed on a notebook or a desktop functions by sending requests to the OS for different services via an application program interface (API). The contact with the OS can be achieved via a Graphical User Interface (GUI) or a command line. In the hosting world, the OS is what runs a web hosting server as well as any software installed on it, which includes not only website scripts, but also 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.