Dan found a very good article about opacity and fill…check it..thanks Dan!

Opacity vs. Fill

Last updated Apr 8, 2005.

When you have a layer selected,
you’ll notice that you have two sliders available for your use at the
top of the Layers palette. One, which you’re probably familiar with,
is Opacity and the other, which you might not be familiar with,
is Fill. While opacity adjusts how opaque a layer—and all of
its contents—are, Fill adjusts just the opacity of filled shapes in
the layer. The minor—but important—difference between the two is
while layer effects are effected by layer opacity, fill doesn’t effect
them at all. This feature is tremendously useful in many respects. The
following steps will demonstrate this feature so you can have a better
understanding of how it works:

1. In Photoshop, press
   Cmd + N (Mac) or Crtl + N (Windows) to open the New
   document window. From the Preset pull-down menu, choose 640
   x 480. This auto-fill the new document settings with one that is
   640 x 480, RGB, and at 72 dpi. Click OK.
2. Create a new layer.
3. Select the Rectangular
   Marquee tool and draw a rectangle. Choose a foreground color (in
   this example I chose red) and fill the rectangle with that color. An
   easy way to do this is to press the keyboard shortcut Option + delete
   (Mac) or Alt + backspace (Windows). This will fill your selected
   rectangle with the foreground color that you’ve chosen.

Figure 1
Rectangle With Fill

1. Deselect your selection
   by pressing Cmd + D (Mac) or Ctrl + D (Windows).
2. Add a Drop Shadow
   layer style to the layer you just drew your rectangle in. In the
   Layer Style dialog box, leave all the Drop Shadow settings at their
   defaults and click OK.

This will add a drop shadow
to the rectangle shape in your image. So far, so good.

1. Lower the Opacity
   of Layer 1 to 50%.

Figure 2
Changing Layer Opacity

1. Duplicate Layer
   1 by dragging the layer to the Create a New Layer icon at the
   bottom of the layers palette. This duplicate layer will automatically
   be titled Layer 1 copy.
2. Select the Move
   tool and while holding down the Shift key, press the Down
   Arrow button on your keyboard about 10 times so that the
   duplicated rectangle is beneath the original rectangle in the image.

Figure 3
Duplicating the Rectangle

1. Select the
   Background layer and create a new layer. This new layer will
   be created beneath the other two layers in this document that you created
   previously.
2. Select a new foreground
   color (in this example, I chose green) and—using your rectangular
   marquee tool—draw another rectangle behind both red rectangles in
   the image. Once you’ve drawn this new rectangle, fill it with your foreground
   color.

Figure 4
Showing off the Transparency

As you can see, because both
red rectangles are 50% transparent, you can see through them to the
green rectangle in the background. At this point both rectangles look
the same.

1. Deselect your selection.
2. Select Layer
   1 copy in the layers palette.
3. At the top of the
   layers palette, drag the Opacity slider to 100%, and drag
   the Fill slider down to 50%.

Figure 5
Changing the Layers Fill %

As you drag that fill slider
down, you should immediate notice the major difference between opacity
and fill.

Figure 6
Opacity vs. Fill

The rectangle at the top has
50% opacity and 100% fill while the rectangle at the bottom has 100%
opacity and 50% fill. While opacity adjusts the opacity for everything
on a particular layer, fill only adjusts the opacity of fills while
leaving layer styles untouched. In a real-world environment, I find
myself using this feature—to accomplish one particular goal or another—in
nearly every project. As you can see, Fill offers you a great
way of altering the opacity of a layer without effecting layer styles
that you’ve added.