Every PC or web server is run by an Operating System - the low-level program which communicates with the hardware, including peripheral devices such as a mouse and a printer, together with any apps that are installed 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 Operating System. On a server, each application runs within the parameters specified by the OS as well - priority, physical memory, processing time, etc. This goes for both standard website 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 can set up a different software environment on the very same machine.