Every single notebook or desktop or server is run by an Operating System - the low-level software which communicates with the computer hardware, including peripherals such as a mouse and a printer, and any applications that are present 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 server, each application runs within the parameters defined by the OS as well - priority, physical memory, processing time, and so on. This is valid for both standard Internet site scripts and server-side software like a media server. When a virtual server is set up on a physical one, there can be two separate Operating Systems, referred to as guest OS and host OS, so you'll be able to set up a different software environment on a single machine.