Each desktop or notebook or web server is run by an Operating System - the low-level program which communicates with the hardware, including peripheral devices such as a mouse and a printer, and any apps that are present 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 Operating System. On a web server, every single app runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. This goes for both standard website scripts and server-side software like a media server. When a virtual server is generated on a physical one, there can be two different Operating Systems, called guest OS and host OS, so that you can set up a different software environment on a single machine.