Each and every PC or server is run by an Operating System - the low-level program which interacts with the hardware equipment, including peripheral devices such as a mouse or a printer, and any applications that are currently 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 OS. On a server, any application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard site scripts and server-side software like a media server. In case a virtual server is generated on a physical one, there can be two separate Operating Systems, named guest OS and host OS, so that you can set up a different software environment on the very same machine.