In its simplest form, jus is simply the pan-drippings from the roasted meat. In practice, the jus is enhanced by deglazing the pan with stock and then simmering the liquid with mirepoix before straining and serving. But the jus is unthickened, which is what distinguishes it from a pan gravy.
Recipes prepared au jus include roast rib of beef (sometimes called prime rib) au jus. Poultry, lamb and veal can also be served au jus.
A thickened version of jus, called jus lié or (fond lié) is prepared by adding cornstarch or arrowroot to brown stock and then simmering.
See also: Au Jus Recipe