Each and every desktop or notebook or web server is run by an Operating System - the low-level program which communicates with the computer hardware, including peripheral devices such as a mouse and a printer, together with any apps 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 OS. On a server, any application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. This goes for both standard Internet site scripts and server-side software like a media server. When a virtual server is created on a physical one, there can be two different Operating Systems, named guest OS and host OS, so you can set up a different software environment on a single machine.