What is the difference between jus and stock?
Jus generally refers to a sauce or accompaniment, served alongside or on top of some other food.

Stock is a (generally gelatin-rich) broth used during the cooking process, whether as a braising/cooking liquid, or reduced/thickened into another sauce.

There are varying ways to make both; the terminology mainly refers to how they're served or used. In my opinion, "jus" is just a fancified way to refer to a stock served as a final component of a meal rather than an ingredient during cooking.