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, along with any applications that are present 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 Operating System. On a web server, each app runs within the parameters set by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard site scripts and server-side software like a media server. When a virtual server is created on a physical one, there can be two different Operating Systems, named guest OS and host OS, so that you'll be able to set up a different software environment on a single machine.