In the Layers panel, there are two sliders, Opacity and Fill. When you slide either of the sliders, they both change the transparency of the layer, and they both seem to do the same thing. But if the layer has a Layer Style applied to it then the Opacity slider will affect both the original pixels and the effects applied to the Layer Style. Fill will only change the original pixels, and will not affect the layer styles at all.
Note: There are 8 special Blending Modes that behave differently when Fill is adjusted, compared to when Opacity is adjusted. These blending modes are Color Burn, Linear Burn, Color Dodge, Linear Dodge (Add), Vivid Light, Linear Light, Hard Mix, and Difference.