Every single desktop or notebook 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, together with any applications that are installed on your 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 server, every single app runs within the parameters set by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard website scripts and server-side software such as a media server. If a virtual server is set up on a physical one, there can be two independent Operating Systems, referred to as guest OS and host OS, which means that you can set up a different software environment on the same machine.