What is the Adorner Layer?

I attended some WPF training in Chicago and talked with one of the presenters, Karl Shifflett. He give the most simple explanation of what the adorner layer is.

It is a layer on top of controls that allows you to decorate them without affecting their layout.

A simple example might be placing a red border around a textbox to highlight an error. If this is done at the adorner layer you will see the red highlight but it will not affect the position of the controls underneath.