Though similar in nature and even use, timing and tradition set stock and broth apart.
In chef training, the first lesson you learn is that stocks are the building blocks of classic cuisine. Master them and you have the basis for many dishes and sauces at your fingertips.
Bones are what makes a stock a stock and not a broth. Stock bones do not have any meat on them, and the recipe may or may not contain mirepoix (onions, carrots, celery) and/or other flavoring agents like thyme and peppercorns.
Stock is made by gently simmering bones or cartilage in water for many hours, depending on the type of bones, which allows the bone marrow and collagen to be released. This long simmer gives the stock a full-bodied flavor and mouthfeel. Stock is made from chicken, beef, pork, and fish bones. (Fish stock takes a much less cooking time due to the delicate bone structure.)
Broth, on the other hand, is generally made from meat, vegetables, and aromatics and has a lighter body than stock. Though it’s sometimes made with meat still on the bone (as in chicken broth made from bone-in chicken pieces) broth’s flavor comes from the meat itself and it is not cooked as long as stock. Additionally, broth recipes often suggest seasoning with salt where stock is never salted.
A benefit of scratch broth is that the cooked meat can be added back into the final dish, like a chicken noodle soup, for example. However, if you want to use the meat off the bone, you must be vigilant with the timing and be ready to pull out the bone-in or whole chicken just before—or just when—it’s fully cooked for the meat to remain tender and retain any flavor. Otherwise, you will overcook it and risk dry, flavorless and chewy meat.
In many recipes for soups and braises, stock and broth may be interchangeable and it will depend on the chef’s direction. But stews, for instance, benefit from the flavor and thickness of stock.
Also, for a reduction sauce, a beefy or full-flavored chicken stock may be the better option because it will produce a nice consistency without needing additional thickeners. Sure, broth can be used in a sauce as well, yet because it lacks the body of reduced stock it will need to be reduced a lot, and if the broth is highly seasoned, reducing it too far may result in an over-salty sauce.