Every single personal computer or web server is run by an Operating System - the low-level software that communicates with the hardware equipment, including peripheral devices such as a mouse and a printer, together with any apps that are present 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 Operating System. On a server, any application runs within the parameters defined by the OS as well - priority, physical memory, processing time, and so on. This is valid for both standard site scripts and server-side software like a media server. In case 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 a single machine.