The work Critias by Plato was left unfinished due to the death of the author and remains incomplete in its original form. It was intended to be a dialogue discussing the ideal society of Atlantis, providing insight into Plato's political philosophy.
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Timeus is about the structure of the world and similar
stuff.
Critias is about a war between Atlantis and Athens.
You can read more here:
http://en.wikipedia.org/wiki/Critias_(dialogue) and here:
http://en.wikipedia.org/wiki/Timaeus_(dialogue)