Opacity alters the transparency while flow effects how the layer opacity fades between the various layers. Lets say you use a brush at 50% opacity, make identical yet seperate strokes. Then use the same brush at 50% opacity and go over one stroke at flow 100 and the other at 50. Youll see the first stroke with flow at 100 overlap the initial stroke and multiply the overlapped to nearly 100 opacity. If you then look at the second stroke with the flow at 50, you'll notice that the overlaped region is about 75% opacity. This is because you are inhibiting the flow, reducing the default multiplying esque effect. Ive used it almost exclusively with photo touch ups. Its hard to exactly explain but it is intended to be used it in conjunction with the opacity. Try using it at the extremes with each one to see the subtle difference.
Adjusting the flow will control the length and to some extent the tapering of your strokes. I've read about adjusting the flow as well but have never realy tried it. I can see where it might be useful in painting charactes though.