Brush:

All brush tools like Pen, Pencil use a brush to paint on an image.

Double click on the brush icon will popup a Select Brush dialog. You can change your brush here.

Double click on any icon will popup a Brush dialog in which you can change the setting of this brush.

Size: Control brush width and height.

Hardness: Control the smooth of the brush.

Roundness: Control the ratio of roundness.

Angle: If roundness is not 100%, it controls the direction of the brush.

Spacing: Control the space between two paint location

Pressure: If you have pressure sensitive devices (like Wacom tablet), these settings control the brush size and/or opacity change following the pressure change.

This is a sample of a single brush with above setting.

This is a sample image using Pen tool with different type of brushes.