Tomato sauce is a much thicker tomato-based sauce than marinara sauce. Typically, the tomatoes have not only been peeled, but also had the seeds removed. Canned tomatoes don’t work as well in this instance. Tomato sauce has more ingredients than homemade marinara sauce, along with a deeper, richer and more complex flavor.  And, believe it or not, traditionally, tomato sauce was not vegetarian. It was based on the French “mother sauce”, Sauce Tomat, and started with a roux made with pork fat. It’s almost like a gravy. In some areas of the United States, one version is actually called Tomato Gravy. In modern Italy, tomato sauce is called Sugo di Pomodoro. It uses few ingredients, but is more flavorful and complex than the canned tomato sauce we buy in the United States. Pomodoro is delicious, all on its own, and often tossed with pasta or used on pizza or even gnocchi. Again, this tomato-based sauce, on it's own, still lacks the heartiness and creaminess of the bolognese sauce used in Rigatoni Bolognese.