Metaphor is used widely in graphical user interfaces to help set users’ expectations and make the behavior of computers clearer. The desktop metaphor is used to suggest that a computer screen is like a physical desk, with papers and folders to shuffle around and various desk accessories, such as calculators, printers, and notepads. A general physical world metaphor is what allows a beveled border to suggest a button and allows close parallel lines to suggest that something is draggable.Metaphors are also useful techniques for designers to explore representations of concepts and the behavior of interface elements. Designers may also apply wild and unrelated metaphors as a useful brainstorming device. [Usability First, 2003]
A very well-known example is the Desktop Metaphor for user interfaces. (see Wikipedia:Desktop_metaphor).
[Saffer, 2005] Dan Saffer, The Role of Metaphor in Interaction Design, Master's thesis, Carnegie Mellon University, Pittsburgh, USA, 2005.
[Usability First, 2003] Usability First. http://u1st.diamondbullet.com/glossary/main.cgi?function=display_term&term_id=195, viewed at: May 24th, 2003.