An Operating System is the low-level software which controls a PC. It takes care of the way your hardware operates, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between applications, and the prioritization of applications in case several run simultaneously. Every program that's installed on a desktop or a laptop operates by sending requests to the OS for various services using an application program interface (API). The contact with the OS is possible through a Graphical User Interface (GUI) or a command line. In the web hosting field, the OS is what runs a web hosting server along with any software installed on it, which includes not only website scripts, but, in addition every other application for example a VOIP or a game server. In case there are virtual machines created, they work with a guest OS that is run on the server host OS.