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 and a printer, together with any apps that are 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 Operating System. On a web server, each and every application runs within the parameters specified 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. In case a virtual server is generated on a physical one, there can be two different Operating Systems, referred to as guest OS and host OS, so that you will be able to set up a different software environment on the same machine.