What is the difference between Opacity and Fill on a photoshop layer?

When we set the Opacity value for a selected layer, the change applies to the entire layer. This means that any layer styles and blending mode for that layer are also affected.

When we set the Fill value,only painted pixels and vector items are affected,layer styles and blending mode remain same.
i tried to show you the main difference between both, Check this SS.

For example,

you can specify a Fill value and blending mode for a layer and only the fill pixels inside the shape are affected-not the layer styles applied to the layer. If you use the Opacity option to adjust transparency both the shape and effects are affected.i used inner shadow
and stroke as layer style,i chose opacity 5% to show you coz in 0% there is nothing to display.
hope this may clear you confusion.