Every single desktop or notebook or server is run by an Operating System - the low-level software that communicates with the computer hardware, including peripheral devices such as a mouse or a printer, as well as any applications 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 OS. On a server, each application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etc. This goes for both standard site scripts and server-side software such as a media server. In case a virtual server is created on a physical one, there can be two independent Operating Systems, named guest OS and host OS, so you can set up a different software environment on a single machine.