The three words are used loosely in shops and precisely in kitchens.
Stock
Made by simmering bones, and usually aromatics, in water. The point is gelatine extracted from connective tissue, which gives body — a good stock sets when cold. It is normally unseasoned, because it is an ingredient rather than a finished dish.
Broth
Made by simmering meat, with or without bones. It is seasoned and meant to be drunk or eaten as it is. It carries more flavour and less body than a stock of the same meat.
Consommé
A stock or broth clarified until it is transparent, classically by simmering it with a raft of egg white and lean minced meat that traps the particles clouding it. The result is judged on clarity as much as on taste.
Why the distinction is worth keeping
A recipe asking for stock wants body and a neutral base it can season itself. Substituting a seasoned broth changes both the salt level and the texture of the finished sauce, and the substitution cannot be corrected at the end.
