He was told the story of Atlantis by a politician named
Critias.
View page
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.
View page
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)