Peter-Paul Koch on A List Apart - JavaScript Triggers:

"CSS triggers are well known. The class and id attributes allow you to fully control the presentation of your websites. [...]

The behavior layer should function in exactly the same way. We should separate behavior and structure by discarding inline event handlers like onmouseover="switchImages('fearful',6,false)". Instead, as with CSS, we should use triggers to tell the script where to deploy the behavior."