Au jus is French for “with [its own] juice”; jus is the juice itself. 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.
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. Jus can be frozen for six months or longer, but the flavor may suffer after this time.
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. The American 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, (also known as Glace de Viande) to accompany a meat dish.
Jus can also be made by extracting the juice from the original meat and combining it with another liquid e.g.: red wine (thus forming a red wine jus).
A powdered product described as jus is also sold, and is rubbed into the meat before cooking or added afterwards. Powdered forms generally use a combination of salt, dried onion, and sometimes sugar as primary flavoring agents.