Each and every PC or web server is run by an Operating System - the low-level software that communicates with the hardware equipment, including peripheral devices such as a mouse or a printer, as well as any applications 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 server, every single app runs within the parameters specified by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard site scripts and server-side software such as a media server. If a virtual server is generated on a physical one, there can be two separate Operating Systems, named guest OS and host OS, so that you'll be able to set up a different software environment on the same machine.