Opacity & Fill – Layer
Opacity and fill both have very different effects on a layer, but you have to do something special in order to see what that difference is. You will only see the effects of using fill if you have activated some layer FX, such as Drop Shadow or Bevel and Emboss. When using these layer fx, dropping the fill will make what ever is actually on the layer lower in visibility, while leaving the FX just as they were. Opacity is the master switch, and will bring the visibility of everything up or down!
Opacity & Flow – Brush
Using Opacity with your Brush Tool will lower the visibility of the tool, but the effect will not build up on itself. As long as you are holding down your mouse (or pen) button, the opacity of where ever you are painting will not exceed the amount specified. To add more, you must first lift your button, and re-apply it.
Flow allows you to go over the same area and buildup the effect each time you cover. Think of using flow as using a marker – the more you go over a certain area, the more visible the marker will become.