Every desktop or notebook or server is run by an Operating System - the low-level program that communicates with the hardware, including peripherals such as a mouse and a printer, and any apps that are currently installed on your system. Any program input using 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, and so on. This is valid for both standard Internet site scripts and server-side software like a media server. When a virtual server is set up on a physical one, there can be two different Operating Systems, called guest OS and host OS, so you will be able to set up a different software environment on the same machine.