well first of all...

Flat top means exactly that.... the top of the piston is flat with the exception of the valve reliefs...

Inverted dome is the same thing as a "dish" piston....meaning when you look at the top of the piston it actually looks like a small dish ( like a dogs bowl)

A domed piston is one that has a pop-up on it....like the flat part with a mountain on top of it....

I just can't get any more easier than that!

A dish is good for lower compression while a dome is good for higher compression