Every single PC or server is run by an Operating System - the low-level software which interacts with the hardware equipment, including peripherals such as a mouse and a printer, together with any applications that are currently 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 Operating System. On a server, each and every app runs within the parameters established by the OS as well - priority, physical memory, processing time, etcetera. This is valid 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 independent Operating Systems, referred to as guest OS and host OS, so you will be able to set up a different software environment on the same machine.