Ratatouille is a common vegetable Stew that originated from southern France, mainly from the region around Nice (Provence) and so the traditional version is called Ratatouille niçoise in France. It comes from the Occitan word ratatolha which means a roughly made stirred stew and was first used around the 1770s to indicate any simple and rapid vegetable stew. The Provencal version resembles another Provencal dish, the bohémienne which is made only with tomatoes, egg plants and olive oil. The traditional French version simply is a stew with egg-plants, bell peppers, zucchini and tomatoes aromatized by a combination of herbs called the Herbes de Provence. I did an original Ratatouille a couple of years ago and you can find the rough recipe here.
