Each PC or server is run by an Operating System - the low-level software that communicates with the computer hardware, including peripheral devices such as a mouse and a printer, along with any applications that are currently installed 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 OS. On a web server, any application runs within the parameters set by the OS as well - priority, physical memory, processing time, etc. This is valid for both standard Internet site scripts and server-side software such as a media server. When a virtual server is set up on a physical one, there can be two separate Operating Systems, referred to as guest OS and host OS, so that you can set up a different software environment on the very same machine.