Each and every personal computer or server is run by an Operating System - the low-level program that communicates with the computer hardware, including peripherals 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, each and every application runs within the parameters established by the OS as well - priority, physical memory, processing time, etc. This is valid for both standard site scripts and server-side software like a media server. When a virtual server is created on a physical one, there can be two separate Operating Systems, called guest OS and host OS, so that you'll be able to set up a different software environment on the very same machine.