Every single personal computer or web server is run by an Operating System - the low-level program that communicates with the hardware, including peripherals such as a mouse or a printer, and any apps that are currently installed 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, each app runs within the parameters defined by the OS as well - priority, physical memory, processing time, etc. This goes for both standard website scripts and server-side software like a media server. If a virtual server is created on a physical one, there can be two separate Operating Systems, called guest OS and host OS, so you'll be able to set up a different software environment on the very same machine.