Opacity is the entire layer (all pixels) including effects and blend modes, where as fill selects only the pixels of the image on the layer selected without effects or blend modes. A good way to view this is to set up an image of two or more layers, one layer being a selected image smaller than the background. Give it an outer glow by double clicking on the layer to bring up the layer styles menu. Leave the Blend Mode on screen on the outer glow and change the color to white at 100% Opacity. Then Click OK. Now view your image and change the Opacity and then view the Fill Opacity changes. Notice how the Outer Glow does not change when you change the Fill Opacity.

Keyboard shortcuts are as follows: When the layer you want to effect is selected: use the #keypad by pressing a number for the Opacity (8=80%,etc.) Pres Shift and a Number for Fill. In both cases ) will get you 100%. You would have to manually enter 0%. You can also scrub in 10% increments on the Opacity by holding shift and scrubbing.