The Difference Between Opacity & Flow In Photoshop
Knowing how and when to manipulate Flow & Opacity will increase your skill and results in Photoshop more than almost anything else.
The Difference between Brush Flow and Brush Density in Lightroom 4
You may notice that when you use the Adjustment brushes in Lightroom 4, there are two sliders at the bottom of the panel called Flow and Density. They affect the opacity of the brush stroke but in two different ways. When they are combined, Flow and Density allow you to really dial in how sensitive your adjustment brush is going to be.