Each and every desktop or notebook or web server is run by an Operating System - the low-level program which communicates with the hardware, including peripherals such as a mouse and a printer, together with any applications that are present 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 web 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 such as a media server. In case a virtual server is generated on a physical one, there can be two independent Operating Systems, named guest OS and host OS, which means that you will be able to set up a different software environment on the same machine.