what is routed events?

Functional definition: A routed event is a type of event that can invoke handlers on multiple listeners in an element tree, rather than just on the object that raised the event. Implementation definition: A routed event is a CLR event that is backed by an instance of the RoutedEvent class and is processed by the Windows Presentation Foundation (WPF) event system.

http://wpftutorial.net/RoutedEvents.html

When you are first getting started with WPF, you will likely use routed events without even knowing you are using them. For example, if you add a button to your window in the Visual Studio® designer and name it myButton and then double-click on it, the Click event will get hooked up in your XAML markup and an event handler for the Click event will be added to the codebehind of your Window class. This should feel no different than hooking up events in Windows Forms and ASP.NET. It is actually a little closer to the coding model for ASP.NET but more like the runtime model of Windows Forms

A routed event is a type of event that can invoke handlers on multiple listeners in an element tree, rather than just on the object that raised the event. The definition is followed by an example of a WPF node with three buttons.

Routed events are events which navigate up or down the visual tree according to their RoutingStrategy. The routing strategy can be bubble, tunnel or direct. You can hook up event handlers on the element that raises the event or also on other elements above or below it by using the attached event syntax: Button.Click="Button_Click". Routed events normally appear as pair. The first is a tunneling event called PreviewMouseDown and the second is the bubbling called MouseDown. They don't stop routing if the reach an event handler. To stop routing then you have to set e.Handled = true; Tunneling The event is raised on the root element and navigates down to the visual tree until it reaches the source element or until the tunneling is stopped by marking the event as handeld. By naming convention it is called Preview... and appears before corresponding bubbling event. Bubbling The event is raised on the source element and navigates up to the visual tree until it reaches the root element or until the bubbling is stopped by marking the event as handled. The bubbling event is raised after the tunneling event. Direct The event is raised on the source element and must be handled on the source element itself. This behavior is the same as normal .NET events.