Every single notebook or desktop or server is run by an Operating System - the low-level program that interacts with the computer hardware, including peripheral devices such as a mouse or a printer, along with any applications 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 web server, each and every application 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 like a media server. If a virtual server is generated on a physical one, there can be two independent Operating Systems, referred to as guest OS and host OS, so that you will be able to set up a different software environment on a single machine.