In French cuisine, stock is referred to as fond, and it is preferable to use that term. It is generally made of bones and flavored with root vegetables and herbs. It is either brown from roasting these or not, and then is white – fond blanc. This is later refined to a jus by deglazing a frying pan with (usually in butter fryied and strained from excess fat, a mix of fresh, new vegetables, meat trimmings, new herbs, and spices) with strained fond (stock). It is reduced to a sirupy consistency and finished with fortified wines and butter, or cognac. It's not the same as demi-glace because they both have a partial reduction, but demi-glace is the ingredient (reduced fond), and jus is the finished product. Fond is an extraction medium for the jus in this case. You can extract a small amount of this when you roast premium cuts in an oven with vegetables and flavourings. The jelly from well roasted chicken on the bottom of the tray is jus. But it is generally not enough and is generally too salty – so gravy is generally made of it. It is thinned with water or stock and thickened with flour or starch. Jus is the extracted juice of the meat in the simplest possible explanation for this Q, but it can get complicated.