Every desktop or notebook or server is run by an Operating System - the low-level software that interacts with the hardware, including peripheral devices 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, every single application runs within the parameters established 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. If a virtual server is set up on a physical one, there can be two independent Operating Systems, called guest OS and host OS, so that you can set up a different software environment on the very same machine.