Lower values create increasingly flat oval shapes. The Rotation spin box adjusts the angle of rotation of the brush you are designing. Rotation only works if you set an Aspect less than 100. The Softness spin box sets the edge softness of the brush you are designing. A setting of 100 produces an opaque, solid brush.
Lower settings blur the edge of the brush strokes.
When you create a new brush, that brush appears in the Brush Selection Grid in the Paint Palette. You can click on any brush and then on the Delete Brush button in the Paint Palette to delete that brush. You can reset all brushes to the Image Composer defaults by clicking on the Reset to Defaults button in the Paint Palette. This button looks a lot like the Undo button in the Image Composer toolbar, except that it is red. When you push this button, you get warned that you're about to get rid of any custom brushes you designed.