Each and every desktop or notebook or server is run by an Operating System - the low-level program which interacts with the hardware, including peripherals such as a mouse or a printer, along with any apps that are present 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 web server, each application runs within the parameters established by the OS as well - priority, physical memory, processing time, etc. This goes for both standard website 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, named guest OS and host OS, so that you can set up a different software environment on the same machine.