An Operating System is the low-level software that manages a PC. It takes care of how your hardware functions, the communication with peripheral devices including a monitor, a printer or a keyboard, the allocation of memory between applications, and the prioritization of apps in case several run simultaneously. Any program which is installed on a PC operates by sending requests to the OS for different services by using an application program interface (API). The interaction with the OS can be performed via a Graphical User Interface (GUI) or a command line. In the Internet hosting world, the OS is what commands a web server along with any software installed on it, which includes not only website scripts, but, in addition any other application such as a VOIP or a game server. If there are virtual machines created, they work with a guest OS that runs on the server host OS.