Every notebook or desktop or server is run by an Operating System - the low-level program that communicates with the hardware, including peripheral devices such as a mouse and a printer, as well as any apps that are present 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 OS. On a server, any application runs within the parameters specified by the OS as well - priority, physical memory, processing time, etc. This is valid for both standard site scripts and server-side software such as a media server. In case a virtual server is generated on a physical one, there can be two independent Operating Systems, called guest OS and host OS, so that you will be able to set up a different software environment on the same machine.