Both styles Shadow, and Inner Shadow behave in much the same way. Shadow will cast outside of the shape it’s applied to, whilst Inner Shadow will render inside the shape.
Multiple shadows can be applied to a shape where their X and Y position, along with blur radius, spread, and color can be defined. You can also adjust a shadow’s blend mode inside the color popover too.
Note: When zooming in to levels above 400%, shadows with a blur value applied will seemingly disappear from layers in the Canvas. This is due to a performance optimization to ensure documents using lots of shadows will render quickly. This will have no effect on any exported assets, and they will appear with shadows applied as expected.
Editing a shadow’s spread value will in-effect increase the size of the object casting the shadow. Please note, it isn’t possible to adjust the spread of a shadow on a text layer.