The game starts were Age of Empires II: Age of Kings ended. In AOEIII you'll start with the European powers to explore, colonize and conquer the New World. The game will feature stunning scenes, from towering European cathedrals to courageous tribes of Native Americans. The game will also offer spectacular combat scenes with Industrial Age units like infantry, cavalry and tall ships bristling with cannons.
Age of Empires III will feature new game-play elements like the new concept of a "Home City", new civilizatons, units, technologies and an extensive single-player campaign that will cover three generations.
Here are some pretty looking screenshots from AOEIII. The game looks real sweet.