Each and every personal computer or server is run by an Operating System - the low-level software which interacts with the hardware, including peripheral devices such as a mouse and a printer, together with any applications that are currently installed 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 Operating System. On a server, every single app runs within the parameters established by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard site scripts and server-side software such as a media server. If a virtual server is created on a physical one, there can be two separate Operating Systems, named guest OS and host OS, which means that you'll be able to set up a different software environment on a single machine.