One of the two types of Object Oriented scripts available in Director. The other type is the Parent Script. The only technical difference between a behavior and Parent Script is that behaviors appear in the Behavior Inspector during authoring, and that a bevahior can be dragged and dropped onto a sprite or the Stage
From a non-programmer's perspective, a behavior is something you can attach to a sprite or insert in a frame (in the scripting channel) to make the sprite or frame act in a specific way. Behaviors are one of the most powerful features of Director, owing to the fact that they are configurable, highly re-usable. Also, more than one behavior can be stacked onto the same sprite, allowing it to perform different stock actions at once.