Each desktop or notebook or server is run by an Operating System - the low-level software which communicates with the computer hardware, including peripherals such as a mouse and a printer, along with 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 web server, each and every application runs within the parameters set by the OS as well - priority, physical memory, processing time, and so on. This is valid 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, called guest OS and host OS, which means that you can set up a different software environment on a single machine.