Every desktop or notebook or web server is run by an Operating System - the low-level program that interacts with the computer hardware, including peripherals such as a mouse or a printer, as well as any apps that are currently 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 application runs within the parameters established by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard Internet site 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 separate Operating Systems, called guest OS and host OS, which means that you'll be able to set up a different software environment on the same machine.