Each and every notebook or desktop or server is run by an Operating System - the low-level program which interacts with the hardware, including peripherals such as a mouse or a printer, as well as any applications 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 Operating System. On a server, each application runs within the parameters set by the OS as well - priority, physical memory, processing time, and so on. This goes for both standard site 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, referred to as guest OS and host OS, so that you will be able to set up a different software environment on the very same machine.