Brushes are convex polygonal surfaces. They can have an associated texture which can either be drawn filling their area or as a sprite. The sprite can be displaced independently of the brush surface.  
Brushes can also be assigned a path that describes how it moves in the bidimensional space and that can be edited with the Path tool.  
Finally, brushes have a built-in property, `collision`, which determines whether they should represent a clipping surface or not. It can be edited in the properties window.