Terms for public eating places in Italy follow what would appear to be a pattern, but since the categories aren't official, the names can be deceptive.
Ristorante should be a full-fledged restaurant providing complete menus (fixed price or à la carte) cooked by a professional kitchen staff and served by waiters, including a sommelier, experienced with foods and wines. The term, from the French restaurant, came into use after the Risorgimento to describe elegant and sophisticated dining establishments. But, as popularity spread, ristorante also came to apply to pretentious ordinary places.
Trattoria, which suggests familiarity as a derivative of trattare (to deal with or attend to), applies to a neighborhood, small town or rural eating house, often family run, serving local foods and wines. Though the surroundings and service are usually unostentatious, like the price, the classic trattoria should provide exemplary regional cooking. Daily menus are often hand written or chalked on a blackboard or simply recited.
Osteria, from the Latin hospes, originally defined an inn providing food and lodging. But the name came to signify a modest wine house, often serving simple foods-like the similarly cozy taverna or locanda. Such locales have faded. Osteria (or hostaria) suggests simplicity, but the term (like locanda, taverna or trattoria) may apply to a sophisticated eating place.
Pizzeria, the pizza parlor popularized in Naples and the south, provides its specialty baked by a pizzaiolo in a wood-fired oven to be eaten on the premises or taken out. As the most popular type of eatery in Italy, the pizzeria no longer confines choices to pizza, but often provides other dishes, usually at lower prices than a ristorante.