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 flavour, 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 flavourful 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 sautéed 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.
In countries such as Britain, Australia, New Zealand and in South Africa the term 'tomato sauce' is used to describe a condiment similar to that known in the USA as 'ketchup'.