Every single notebook or desktop or server is run by an Operating System - the low-level software which interacts with the hardware, including peripheral devices such as a mouse and a printer, along with any apps that are installed on the 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 OS. On a server, each application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard website scripts and server-side software like a media server. In case 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 the same machine.