Each and every personal computer or server is run by an Operating System - the low-level program which communicates with the hardware equipment, including peripheral devices such as a mouse or a printer, and any apps that are present on the 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 server, every single app runs within the parameters defined by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard site scripts and server-side software like a media server. In case a virtual server is set up on a physical one, there can be two independent Operating Systems, named guest OS and host OS, which means that you'll be able to set up a different software environment on the very same machine.