Each and every notebook or desktop or server is run by an Operating System - the low-level software which interacts with the computer hardware, including peripherals such as a mouse and a printer, as well as any apps that are installed on your system. Any program input through a command line or a Graphical User Interface (GUI) is processed by sending an Application Program Interface (API) request to the Operating System. On a web server, every single app runs within the parameters specified by the OS as well - priority, physical memory, processing time, etc. This is valid for both standard Internet site scripts and server-side software such as a media server. If a virtual server is created on a physical one, there can be two independent Operating Systems, referred to as guest OS and host OS, which means that you can set up a different software environment on the same machine.