Shadow (outer) and Inner Shadow have the same settings and work in the same way except that one is inside and the other is outside the shape.
Each Shadow can have its own blending as well, and this can be configured in the color popover.
Each shadow also has a spread value which can be used to to in-effect grow the object that is casting the shadow.
Note: an Inner Shadow on a text layer looks best if the blur radius is set to zero. Spread is also unavailable for text layers.