In cooking, a sauce is liquid or sometimes semi-solid food served on or used in preparing other foods. Sauces are not consumed by themselves; they add flavor, moisture, and visual appeal to another dish. Sauce is a French word, meaning salted. Sauces need a liquid component, but with dishes such as pasta can contain more solid elements than liquid.
Sauces may be prepared sauces, such as soy sauce, which are usually bought, not made, by the cook; or cooked sauces, such as Béchamel sauce, which are generally made just before serving. Sauces for salads are called salad dressing.
Another variation is the pan sauce; this is made by adding an aromatic (such as chopped shallot) to a pan that has previously cooked meat, which has left hardened juices (called the fond) in the pan. After the aromatic has softened, a liquid (such as stock, wine, or water) is added to melt the fond in the bottom of the pan (a process called deglazing). Butter can then be added to this to make a quick sauce.
There are also many sauces based on tomato (such as tomato ketchup and tomato sauce), other vegetables and various spices.
Sauces can also be sweet, and used either hot or cold to accompany and garnish a dessert. Another kind of sauce is made from stewed fruit, usually strained to remove skin and fibers and often sweetened. Such sauces, including applesauce and cranberry sauce, are often eaten with specific other foods (apple sauce with pork, ham, or potato pancakes; cranberry sauce with poultry) or served as desserts.
(by the Wikipedia)