A "command line interface" or CLI is a method of human-computer interaction that receives lines of commands in written form either from keyboard input or from a script and generally outputs only text to the display. Examples include the Unix shell, VMS DCL, and related follow-on designs like CP/M and MS-DOS.
Contrast with Graphical user interface. Even though new users seem to learn GUIs more quickly, CLIs are still important because some of the more powerful CLIs double as scripting languages (see shell script) and can perform operations in a batch processing mode without user interaction.
See (as soon as pssible, IMHO): "In the Beginning Was the Command Line", by Neal Stephenson http://www-classic.be.com/users/cryptonomicon/beginning_print.html