Au jus [oh joos] is French for ‘with [its own] juice.’ In American cuisine, the term is mostly used to refer to a light sauce for beef recipes, which may be served with the food or placed on the side for dipping. In French cuisine, jus is a natural way to enhance the flavor of dishes, mainly chicken, veal and lamb. ‘Jus’ means the natural juices given off by the food. To prepare a natural jus, the cook may simply skim off the fat from the juices left after cooking and bring the remaining meat stock and water to a boil.
Often prepared in the United States is a seasoned sauce with several additional flavorings. American recipes au jus often use soy sauce, Worcestershire sauce, salt, pepper, white or brown sugar, garlic, onion, or other ingredients to make something more like a gravy. So-called jus is sometimes prepared separately, rather than being produced naturally by the food being cooked. An example could be a beef jus made by reducing beef stock to a concentrated form, to accompany a meat dish.
The Daily Omnivore 