Every single notebook or desktop 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, together with any apps 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, any application runs within the parameters specified by the OS as well - priority, physical memory, processing time, and so on. 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 independent Operating Systems, referred to as guest OS and host OS, so you will be able to set up a different software environment on a single machine.