Clicking in the icon on the brush settings panel will bring up the patterns panel.

There are many uses for the patterns panel.  The basic function is to have one-click access to a number of images that can be loaded into a custom brush.

Once a brush, these images can be stamped down, used as a fill pattern, warped into a shape, or used like a brush.

You can also save your current brush as a pattern, using the 'Save current brush as pattern' button.

Here are a few ways a pattern can be warped into various shapes.  See the section on the fill settings panel for more information.