Every desktop or notebook or server is run by an Operating System - the low-level program that interacts with the computer hardware, including peripheral devices such as a mouse or a printer, and any applications that are installed 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 OS. On a server, each and every app runs within the parameters set by the OS as well - priority, physical memory, processing time, etc. This goes for both standard website scripts and server-side software like a media server. If a virtual server is generated on a physical one, there can be two independent Operating Systems, called guest OS and host OS, so you can set up a different software environment on the same machine.