Each personal computer or web server is run by an Operating System - the low-level program that communicates with the hardware, including peripheral devices 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 OS. On a web server, every single app runs within the parameters set by the OS as well - priority, physical memory, processing time, etc. This goes for both standard site scripts and server-side software like a media server. If a virtual server is set up on a physical one, there can be two separate Operating Systems, referred to as guest OS and host OS, so you will be able to set up a different software environment on the very same machine.