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