Classification and use of Stocks vs. Broth:
Broths are generally the result of preparing another item and usually not prepared specifically on their own. The juices poured off from a roasted turkey (after being degreased) would be considered broth. Whole chickens being poached for another preparation would create broth.
Stocks are prepared specifically for use in other recipes (sauces, soups, stews, rice, etc.) Stocks are never salted in their preparation or the finished dish will most likely end up too salty due to reduction that will take place upon further cooking.
Stocks are usually simmered for a very long time (4-6 hours for chicken & 8-12 for veal/beef) to extract maximum flavor and gelatin from the bones.
Broths aren't usually cooked nearly as long due to the fact that cooking the meat for extended periods (even chicken surrounded by the liquid) will result in tough, flavorless meat.
Consomme: a fortified and clarified stock. The stock is fortified in flavor by the addition of a "raft" which is a combination of lean ground meat (appropriate to the type of stock being used) with brunoise (1/16 inch) mirepoix (carrots, onions, celery), and egg whites. The raft mixture is stirred into the cold stock and as it gently heats, the proteins coagulate forming a "raft" on top of the stock. A small hole is poked in the center (if one hasn't already formed) and as the stock bubbles through the hole it leaches back through the ground meat/egg white raft which filters out impurities to clarify the stock and fortify it with flavor.
Bouillon: French word for broth.
Court Bouillon: sometimes called a "short broth". A poaching liquid usually used for fish that is usually comprised of water, acid (lemon juice, vinegar, wine), parsley stems, bay leaves, peppercorns, and some salt.
When to use Stock vs. Broth:
Use stock when a sauce is to be reduced significantly or when clarity of the final result is preferred.
Broths can be substituted for stock when the body of the liquid or clarity isn't important, and when the liquid will be thickened by addition of a starch.