The flow and opacity settings for the brush tools in Photoshop can be a little confusion. Watch this video (tip: content starts 2 minutes in) from Aaron Nace at Phlearn for a great visual explanation of how the flow and opacity settings work.
The basic idea is that the Flow setting allows you to build up ink over and over again, while Opacity gives you a single value of ink on your brush. These settings come in very handy for dodging and burning on images as the above video demonstrates.
