There's a LOT of heated debate on this one. The three (sauce, gravy, ragu) are often used interchangeably to mean the delicious liquid stuff you put over pasta. Sauce is the most general and encompasses meat-free options like marinara as well as meat sauces. Ragu and gravy both contain meat. You might also hear "Sunday Sauce" or "Sunday Gravy," which contain all kinds of meats like meatballs, braciole, pork chops, and/or sausage.