Every single desktop or notebook or server is run by an Operating System - the low-level program that communicates with the hardware equipment, including peripherals such as a mouse or a printer, together with any applications that are present 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 OS. On a server, each application runs within the parameters established 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. If 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.