The name Plato actually means 'broad shoulders' and I like to think that that wasn't his name at all. It amuses me to think of him meeting people at Ancient Greek parties and such and saying to them "Hey, my name's Broad Shoulders" when really he had a really ridiculous name. But, anyway, that's a little bit of a silly tangent. His name isn't very important, though it is the source of the word 'platonic'.
At the centre of all of Plato's writing is one person: Socrates. Socrates was Plato's teacher and friend, and so Plato was, quite understandably, pretty sad when he was executed. As such, Plato wrote down all of the ideas which came from Socrates, and he did this in the form of dialogues. These dialogues were always pieces of writing where some philosophers would have a discussion with Socrates; Socrates brings up his point of view, and then verbally destroys all of their counter-arguments. The Socrates of these writings was also used as a mouthpiece for Plato's own ideas. Strangely enough, despite the fact that some of these dialogues may well be based on real discussions, Plato is never in them himself, and sometimes they stop and say "Hey, where's Plato?" and somebody will just say "Oh, he's down at the gym working out" or something like that, and then they'd just carry on.
The most important of Plato's philosophical ideas, or at least the best known, is that of the World of Forms, or the Intelligible Realm as he called it. Plato basically argues that there is a higher plane of existence where there is a perfect version of everything and that this is the place where human souls come from. Everything in the real world is a shadow of something in the Intelligible Realm and is not really worth studying (art, he says, is a shadow of a shadow, and so worthless). He believes that this exists, because if somebody is asked to picture, say, a chair, they will be able to do so and this is the perfect chair. He says the reason they can picture it, is because they can remember having seen it before their soul came to Earth, we never learn, merely remember. The most important thing in the Intelligible Realm is the Form of the Good, which is basically perfect morality. Through philosophy, says Plato, we will learn about the Form of the Good. Many believe this to have been inspiration for Christian ideas of Heaven and God.
So that's an outline of Plato. It may also interest you to know that the legends of Atlantis stem from Plato's writing, or at least, that the earliest mention of Atlantis comes from him. It's generally accepted that he just made it up though. When I get round to it, the next philosopher entry will be on John Locke.