Use Icons for Recognizable Concepts

In general, recognizing is considerably less taxing than remembering. Icons quickly stimulate necessary recollections of familiar material. They may be used for labeling (e.g., menu items), indicating (e.g., system state), warning (e.g., error messages), identifying (e.g., file type), manipulating (e.g., resizing), and containing (e.g., trash). The design of an icon can take several forms:

Resemblance: where the image and the concept are related by analogy.		Exemplar: where the image gives an example of the concept.
Symbolic: where the concept is abstracted from the image.		Arbitrary: where the image-concept relation must be learned.