Every personal computer or server is run by an Operating System - the low-level program that communicates with the hardware equipment, including peripheral devices such as a mouse and a printer, along with any apps that are 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 Operating System. On a server, any application runs within the parameters set by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard website scripts and server-side software like a media server. When a virtual server is created on a physical one, there can be two independent Operating Systems, named guest OS and host OS, which means that you will be able to set up a different software environment on the same machine.