Terminal (or Terminal.app) is the program included in Mac OS X for running applications and issuing commands using the command line interface. These programs are controlled by typing lines of text, rather than using the mouse or keyboard commands in the usual graphical user interface (GUI).
This terminal emulator originated in Mac OS X's predecessors, NeXTSTEP and OPENSTEP, and allows the user to interact with the computer through a command line interface. By default it provides a bash shell, which refers to a UNIX command language.
Most users of Mac OS X have no need to use the Terminal, as it provides advanced features not commonly encountered. The Terminal allows the user to manipulate additional parameters of certain applications that are not available through those applications' menu and mouse commands. It is also used to control system parameters and to run certain "hidden" programs that have no graphical user interface whatsoever, are not visible in the Finder, and have no commands that can be accessed through graphical menus or the mouse.