CLI = Command Line Interface and GUI = Graphical User Interface. These two ways of interacting with one’s computer are often thought of as opposites or at least exclusive of each other.
However, in modern operating systems, one can have both simultaneously, with any number of terminal windows surrounded by GUI applications. Various scripting languages and utilities such as QuickSilver give even casual users the ability to bridge the CLI and GUI in new and creative ways.
Strangely enough for those accustomed to flashing lights and zooming windows, the simple, austere nature of the Command Line has its aesthetic appeal as well, with none of the “bells and whistles” typical to the GUI — which, for users trying to minimize distractions, is definitely a bonus.
osnews.com: The Command Line - The Best Newbie Interface?

