When you click on a link in a component, that component receives an event which it will normally pass on to its listener, which is a controller.
Example: a click in a menu/tree is first dispatched to the window and then automatically to the Menu Component. The Menu Component will forward a "node selection" event to its controller. The controller then does what makes sense to fulfill its business logic (e.g. fill the content with the Settings tab when the user clicked on the "settings" node)
Look at the code /javasrc/SimpleDemoController.java.html how event-handling is done.