Plato was a disciple of Socrates, accepting his basic philosophy and dialectical style of debate: the pursuit of truth through questions, answers, and additional questions.
At the heart of Plato's philosophy is his theory of Forms, or Ideas. Plato believed that there exists an immaterial Universe of 'forms', perfect aspects of everyday things such as justice, smallness, greatness, table, bird, various actions, and so on. The objects and ideas in our material world are shadows of the forms. For example, although no two chairs are exactly alike, they both share the same "chairness" that defines them as a chair. Plato conceived the Forms as arranged hierarchically; the supreme Form is the Form of the Good which illuminates all the other Ideas.
All knowledge comes from these forms and philosophy (the pursuit of these forms), while nothing of any value comes from the material world (bodily pleasures, including love, are bad, in that they inhibit the pursuit of knowledge of the forms). And furthermore, we know of these forms because we knew them before birth -- we forgot them upon birth, and we will rejoin them after death. If we live a life full of sensual pleasure, our immortal souls will become lost, and we will come back as lower animals. If we are good philosophers, we will go to join God for eternity...
Plato's stuff is crazy, but at the time, it was a very well put together worldview. If we can believe the dialogs, the wisdom of Socrates (Plato's writings are supposed to be reports of discussions that Socrates and a few other wise men used to convince others of their views) was entirely convincing in their day. Some of it is still of use (Plato/Socrates attack on democracy is still one of the best, and variations of his view of the forms is still used as one possible interpretations of universals). Most of it seems a little silly, in part because of changing worldviews and basic assumptions, and also because the dialogs rely way to heavily on arguments from analogy.
No one knows which stuff was written when, but here's a basic grouping. The question marks indicate that some (?) or many (??) scholars claim that a work is not truely Plato's.
Dialogues I haven't been able to find 'chorological estimates' for.
- Early dialogues
- Middle-period dialogues
- Late dialogues
Thanks to purple_curtain for ier help with the list of works.
The 'question mark' rankings come from http://eawc.evansville.edu/essays/suzanne.htm