I can give you a quick explanation of how they are:
Final Fantasy I
The world has crumbled, it is up to the four heroes of light to save it and restore it.
Final Fantasy II
Diplomacy and war, your a part of a rebellion, going against an dark and evil emperor set to conquer the whole world to do his bidding through the most evil of ways.
Final Fantasy III
Same as Final Fantasy I
Final Fantasy IV
More personal and engaging story, they still have to save the world in the end as everything get's more advanced and complex.
Final Fantasy V
Kinda like I and III, but with more character development and the awesomeness of GILGAMESH!
Final Fantasy IX
Uhh... I don't know how to explain it yet. T_T