Every notebook or desktop or server is run by an Operating System - the low-level software which interacts with the computer hardware, including peripherals such as a mouse or a printer, along with any apps that are installed on your 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, every single app runs within the parameters established by the OS as well - priority, physical memory, processing time, etcetera. This goes for both standard Internet site scripts and server-side software like a media server. When a virtual server is generated on a physical one, there can be two different Operating Systems, named guest OS and host OS, which means that you will be able to set up a different software environment on the very same machine.