It is a brush that draws fur.
The brush works as you've likely assumed, just draw where you want the fur to go. It is very useful for comics and other such works.
* Use the brush on layer(s) clipped to the fur layer to create shadows and highlights.
* Works well with the "Layer Color" effect located in the "Layer Properties" panel.

* Works well with the "Border" effect located in the "Layer Properties" panel.

Example:

Category 1