Each and every notebook or desktop or server is run by an Operating System - the low-level program that interacts with the computer hardware, including peripherals such as a mouse and a printer, along with any applications that are installed on your 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, every single app runs within the parameters established by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard Internet site scripts and server-side software like a media server. If a virtual server is created on a physical one, there can be two separate 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.