Ratatouille started out as a cheap home-cooked meal in the south of France, where these vegetables grow easily in the warm summer sun. Families would simmer them together in a big pot. It was peasant food - the kind of dish you made because the vegetables were ready in the garden.
Today there are two main ways of making it. The 'rustic' way is to chop everything up and stew it all together, so the flavours melt into each other. The 'fancy' way - made famous by a Disney film of the same name - is to slice the vegetables into thin rounds and arrange them in a beautiful spiral pattern in the dish.
The dish is named after the French word touiller, which means 'to stir' or 'to toss'. So ratatouille really means something like 'a stir of bits and pieces'. The name fits - it really is just lots of good things stirred together.
It is usually served warm, with bread or rice. In the south of France, lots of families make a big batch and eat it for several days, because it actually tastes even better the day after you cook it - as the flavours have more time to mix.