Every desktop or notebook or server is run by an Operating System - the low-level software that interacts with the hardware equipment, including peripheral devices such as a mouse or a printer, along with any applications that are currently 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 server, every single application runs within the parameters specified by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard site scripts and server-side software like a media server. If 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 will be able to set up a different software environment on a single machine.