Each and every PC or server is run by an Operating System - the low-level program that interacts with the hardware, including peripheral devices such as a mouse and a printer, as well as any applications 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 web server, each app runs within the parameters set by the OS as well - priority, physical memory, processing time, etcetera. This goes for both standard site scripts and server-side software such as a media server. When a virtual server is generated 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.