In Photoshop, what's the difference between "Fill" and "Opacity"?

So in terms of layers, what's the difference between, say, 50% fill and 50% opacity? I use photoshop 7 by the way.

In Photoshop, what's the difference between "Fill" and "Opacity"?network

The difference is that opacity affects all of the layer including layer effects. Fill only affects the layer content, but not layer effects.