Every single notebook or desktop or server is run by an Operating System - the low-level program which interacts with the hardware equipment, including peripherals such as a mouse or a printer, as well as any applications that are currently installed 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 OS. On a server, every single app runs within the parameters established by the OS as well - priority, physical memory, processing time, etcetera. This goes for both standard website scripts and server-side software like a media server. When a virtual server is created on a physical one, there can be two separate Operating Systems, called guest OS and host OS, so that you can set up a different software environment on the same machine.