Button Styles

What difference am I suppose to notice between these two button styles:  no border, flat button?  They seem to look and behave the same to me.

Flat has a roll-over image.
Right-click on a button, properties, Button Style for a description of each style

They both appear to have the same rollover effect with a raised button outline, but if you're saying one will display another image if provided, that isn't something I've tried yet.