Metaphors are a technique used to simplify interface design. A carefully chosen metaphor can assist a user new to a particular interface. One of the most common and successful metaphors is the desktop, files, and folders. Comparing a computer's system for organizing textual documents to a desk and filing cabinet helps a user picture how their files are being stored. A text-driven command line interface offers no metaphor, making it more difficult for a beginner user. There are folders which hold other folders, or files. There is a trash can to delete files.
The usefulness of a metaphor can only extend so far. Even the popular desktop metaphor breaks down for certain actions. How would a desktop represent ejecting a disk? Windows offers no desktop metaphor for this action requiring users to use a contextual menu. The Macintosh gives the trash can "magic" abilities, allowing the user to eject a disk by dragging its icon on top of the trash icon.
Apple's new Mac OS X uses a traffic light metaphor to help users determine the function of its close, minimize, and expand buttons. The green button expands, the yellow button minimizes, and the red button closes. But even this simple metaphor suffers from several disadvantages. Most traffic lights are oriented vertically, while Mac OS X positions buttons horizontally. Color blind users cannot take advantage of the assistance the colors provide. Furthermore, some localities might not be familiar with street lights or use shapes along with colors.
Sometimes metaphors are simply difficult to use. See if you prefer the knob or slider in the example below.