Living from 427/428 BC to 348/347 BC, Plato was the first in a long line of great thinkers to be influenced by the philosopher Socrates. His most notable work is the Republic. He is also one of two (the other being Xenophon) direct sources for studying Socrates' life as Socrates did not spend time documenting himself. Plato explained many of his philosophies through play-like dialogues. He often used Socrates as one of his characters.
It is tough to say where the nonfictional and fictional writings of Socrates split. Although Plato had begun writing Socrates' stories as history (in the Apology, Crito, and Phaedo), he later developed the character mainly to share his own ideas (the Republic, Timaeus and Critias [he discusses Atlantis a little in Timaeus and even more in Critias]).
One of Plato's most important ideas is the idea of forms. He says that everything in this world is based off of a form or idea elsewhere. The reason we know a chair from a tree is because of our ability to recognize the chair's form. Yet, it is impossible to create a perfect form in our terrestrial plane (because it could not include every type of chair in its single physical aspect). Aristotle, on the other hand, though that everything's essence was found in the object itself (almost the opposite of Plato).
Plato's works include: