Each notebook or desktop or web server is run by an Operating System - the low-level program which interacts with the computer hardware, including peripheral devices such as a mouse and a printer, together with any apps 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 OS. On a web server, any app runs within the parameters set by the OS as well - priority, physical memory, processing time, etc. This goes for both standard site scripts and server-side software such as a media server. When a virtual server is generated 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 the same machine.