What's the difference between a trattoria, an osteria and a ristorante?

Historically: an osteria served wine with simple food, a trattoria was a family place with a short home-cooked menu, and a ristorante was formal and more expensive. Today the words are used loosely and a fashionable spot may call itself an osteria while charging ristorante prices. The useful signal isn't the word on the sign, it's the menu: short, seasonal and local means good, regardless of the label. Two other words worth knowing — a tavola calda is a cafeteria for a quick cheap lunch, and an enoteca is a wine bar that often serves excellent small plates.