A tomato sauce is any of a very large number of sauces made primarily out of tomatoes, usually to be served as part of a dish (rather than as a condiment). Tomato sauces are common for meat and vegetables, but they are perhaps best known as sauces for pasta dishes.
Tomatoes have a rich flavor, low liquid content, very soft flesh which breaks down easily, and the right composition to thicken up into a sauce when they are cooked (without the need of thickeners like roux). All of these qualities make them ideal for simple and appealing sauces.
The simplest tomato sauces consist just of chopped tomato flesh (with the skins and seeds optionally removed), cooked in a little olive oil and simmered until it loses its raw flavour, and seasoned with salt.
Water (or another, more flavorful liquid such as stock or wine) is often added to keep it from drying out too much. Onion and garlic are almost always sweated or sauteed at the beginning before the tomato is added. Other seasonings typically include basil, oregano, parsley, and possibly some spicy red pepper or black pepper. Ground or chopped meat is also common.
Most often, Italian tomato sauces can be switched with more authentic white sauces; cavatelli is best served with traditional Italian white sauces (consisting of mostly fresh parmesan and cream), and many other traditional ingredients. Some Italian Americans on the East Coast refer to tomato sauce as "gravy", "tomato gravy", or "Sunday gravy", especially sauces with a large quantity of meat simmered in them, similar to the Italian Neapolitan ragù. "Gravy" is the literal English translation from the Italian sugo which means sauce.
Marinara is a US-American-Italian term for a simple tomato sauce with herbs—mostly parsley and basil—but, contrary to its name (which is Italian for coastal, seafaring) without anchovies, fish or seafood. In other countries marinara refers to a seafood and tomato sauce.
American supermarkets commonly carry a variety of prepared tomato sauces described as "spaghetti sauce" or "pasta sauce". Common variations include meat sauce, marinara sauce and sauces with mushrooms or sweet red peppers.
A spicy tomato sauce known as sauce piquante is common in Louisiana Cajun cuisine, that can contain any seafood, poultry, or meats such as wild game. It is typically served over white rice. In Louisiana Creole cuisine, there is a tomato sauce known as a creole sauce. It is similar to Italian tomato sauce, but features more Louisiana flavors derived from the fusion of French and Spanish cooking styles. They both usually contain the traditional holy trinity of diced bell pepper, onion, and celery.
Tomato gravy, which is distinct from the term as used by northeastern Italian Americans when referring to tomato sauce, is a gravy common in most rural areas where tomatoes were a staple food. Tomato gravy is prepared in a method similar to white gravy. The cooked tomatoes, some fat (usually cured pork fat) and flour are cooked together until thick, and seasoned with salt and pepper. Typically, tomato gravy is served over eggs, toast and biscuits.