Each and every personal computer or server is run by an Operating System - the low-level program that interacts with the computer hardware, including peripheral devices such as a mouse or a printer, and any apps that are currently installed 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 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 website 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 will be able to set up a different software environment on the very same machine.