Stock and broth are more or less the same thing, a mixture of any combination of meats (including poultry or seafood), bones, vegetables or herbs simmered in a large quantity of water, then strained.
You can use either stock or broth for keeping dressing moist or as a basis for gravy, but a strong flavor will give you better results. The strength of the flavor depends on how the mixture was prepared: the quantities and proportions of the ingredients, amount of water used, whether the meats or bones or vegetables were roasted before simmering, and how long the mixture was simmered.
Chefs rely on stock as an ingredient in itself: the basis for other soups, stews, gravies and sauces, using it instead of water for steaming, even putting it into salad dressings and co*cktails. Stock can be weak or strong in flavor, depending on what the chef requires and how much effort is put into making it. The cans or cartons of broth that we see in the markets have a medium-strong flavor that makes them suitable for serving as a finished soup.
When you see a recipe that calls for "stock or broth," what it is really saying is "homemade or canned." Which one you choose depends on your inclinations. Though I keep a supply of homemade stock in my freezer, I often use broth from cans or cartons (usually labeled broth but, confusingly, may also be labeled stock) for my everyday cooking. Canned broth can be simmered to reduce it and strengthen the flavor, if you think it's a bit too weak for your purposes. The best trick to boosting flavor from canned chicken broth is to simmer the bones of a (purchased) roasted chicken in it for an hour.
At Thanksgiving, though, I make my own stock. I always roast two turkeys, preparing a small one the day before Thanksgiving. After removing the meat from that turkey (setting it aside for leftover sandwiches), I simmer the bones with herbs and vegetables to make a strongly-flavored stock for my favorite gravy, Mark Bittman's Make-Ahead Gravy. If that seems extreme — and perhaps it is — turkey broth/stock from the supermarket will work perfectly well.
