In his last article called TextBox Magic, Jeremy Likness showed how to create a dependency property to enable a textbox filter that would prevent anything but digits. In this article, he takes it a step further and turns it into a "true" behavior.
First, the behaviors can integrate with Expression Blend. You can define a behavior, import it into the tool, and literally drag it onto a control to attach the behavior. Second, behaviors automatically provide overrides to tap into the attach/detatch events. Third, behaviors are typed to their target controls: you may have a generic behavior that targets FrameworkElement or a specific behavior that targets TextBox. Finally, we can decorate behaviors with attributes that are discovered by Intellisense and available when attaching the behavior.