Every PC or server is run by an Operating System - the low-level program that communicates with the hardware equipment, including peripheral devices such as a mouse or a printer, and 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 server, each and every application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etc. This goes for both standard Internet site scripts and server-side software such as a media server. When 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, which means that you can set up a different software environment on a single machine.