Each desktop or notebook or server is run by an Operating System - the low-level software that interacts with the hardware, including peripheral devices such as a mouse and a printer, as well as any apps that are installed on the 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 server, each and every app runs within the parameters specified by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard website scripts and server-side software such as a media server. In case a virtual server is set up on a physical one, there can be two independent Operating Systems, called guest OS and host OS, so that you will be able to set up a different software environment on the very same machine.