The Brush Tool is designed to build tone as you draw over and over the same area in one session. The Non-Building brush on the other hand, does not build tone when you paint over the same area with the same brush stroke. This can be particularly useful when drawing masks and such like as you will always end up with a smooth, flat finish. 

To apply the non-building brush characteristics to all brushes, switch on the option at the top right of the Brushes Library, or alternatively only apply it to selected brushes in the Brush Editor.