- Realistic/ Representational: Recognizable, simplified images of an object or an action. Most of the icons in the real world are developed around this idea.
- Abstract/ Representative: Highly simplified images in which the original object or action is reduced to graphical elements only. An example of showing thermometer as heat - heat does not equal thermometer. It's a representation of heat; Red = Stop;
- Arbitrary/ Symbolic: are invented graphical constructions with no reference (except, possibly conceptual) to actual objects or actions. Database = Cylinder (I haven't seen a DB that looks like a cylinder)
A good reference and study material on icons: http://www.indiana.edu/~vdim/Icons/1Intro/intro.htm