The Paint Brush tool is similar to the pencil, but has more features. It too can be used in different colors, but the shape and size of the Paint Brush can also be changed. You can use square, round, and slanted shaped brushes, of various sizes, selected in the Toolbar Options.

Example:
  You can use the various paintbrush shapes and sizes to create a variety of effects.  Draw thick freehand lines like you would with the pencil, or fill in color in unbounded areas.

Tip:  If you hold down Shift as you paint with the Paint Brush tool, you can make perfectly straight horizontal or vertical strokes.  However, unlike the Pencil, you cannot do 45 degree angle diagonal lines with this method.