Plato was decendend from a moderately well off family. His ancestor, Glaucon, was one of the most well known members of the Athenian aristocracy.
Plato was deeply influenced by the Pythagoreans, whose notions of numerical harmony have clear echos in Plato's notion of The Forms. See Platonic realism (actually, there should be a section of the Plato article that explains about Platonic realism).
there should be a whole section on the dialogue form
One of Plato's legacies, and perhaps his greatest, was his dualistic metaphysics. He saw the world, and the things in it, as imperfect copies of the intelligible Forms or Ideas. These Forms are unchangable and perfect, and are only comprehensible by the use of the intellect, the mind. This idea would inspire later thinking such as Neoplatonism (see Plotinus) and Gnosticism.
His best known book was The Republic, but he is also well known for Laws and his his many dialogues featuring Socrates. He also began the discussion of Atlantis by mentioning it in his Timaeus and Critias.
See the Complete works of Plato.
Obviously, we need to say a lot more about Plato...
"The safest general characterization of the
European philosophical tradition is that it
consists of a series of footnotes to Plato"
-A. N. Whitehead, Process and Reality, 1929