Every 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 apps that are present on your 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 Operating System. On a server, every single app runs within the parameters specified by the OS as well - priority, physical memory, processing time, etc. This goes for both standard website scripts and server-side software such as a media server. When a virtual server is generated on a physical one, there can be two separate Operating Systems, called guest OS and host OS, so that you can set up a different software environment on the same machine.