Each notebook or desktop or web server is run by an Operating System - the low-level software that interacts with the hardware, including peripheral devices such as a mouse and a printer, together with any applications that are currently installed on your 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 Operating System. On a server, each application runs within the parameters established by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard Internet site scripts and server-side software like a media server. If a virtual server is set up on a physical one, there can be two different Operating Systems, called guest OS and host OS, so that you'll be able to set up a different software environment on the same machine.