Ratatouille is a vegetable stew made popular in the Provence region of France. "Rata" is a slang term for stew in French, and "touiller" means "to stir". So literally, a stew that is stirred.
There are many variations to ratatouille, and no one standard or correct way to prepare this dish. Think of it as similar to American "stew", it can take on a number of forms and is open to your interpretation.
This ratatouille variation includes mushrooms for some additional succulent texture and balsamic vinegar for a rich flavor with a touch of tangy sweetness.