Water is the most versatile of cooking liquids, but that's precisely because it adds no flavor. At times you might prefer to use a liquid that adds flavor and body to your finished dish, such as a stock or broth. To non-professionals, the terms are interchangeable, but commercial kitchens recognize a distinction between the two. In theory they're differentiated by their basic ingredients, but in practice it boils down more to their intended use.
Theory and Practice
As laid out by generations of culinary textbooks, bones create the basis for stock. In the case of a chicken stock, this typically includes the necks and backs and sometimes the wingtips. These are all rich in natural gelatin, which gives the stock its body. Chicken broth, on the other hand, is more likely to use a whole chicken or bone-in chicken pieces. This blurs the theoretical distinction, since there is some meat in the stock and some bone in the broth. In practice, the difference is largely at the chef's discretion. If the finished liquid is left unseasoned, for use as an ingredient in other preparations, it's a chicken stock. If it's fully seasoned and flavorful in its own right, it's considered a broth.
The Basic Technique
Chefs rely on a weight-based ratio to reliably produce stocks and broths with a good flavor. For a given weight of cold water, you'd use half that weight of chicken or bones, and 1/10 that weight of aromatic vegetables -- the classic "mirepoix" of onions, carrots and celery. As you bring the water slowly to a simmer -- never a boil -- proteins from the chicken's juices will rise and make an unappetizing grey layer at the top. This should be skimmed away diligently, along with any pools of surface fat, for the clearest and most professional-looking result. A chicken stock typically needs up to 3 hours for its flavors to be fully extracted. If you're making broth rather than stock, you'd season it to taste with salt, pepper and herbs.
Restaurants use large stock pots with a spigot at the bottom, so the finished broth can be drained away without disturbing the bones and meat or stirring up sediment. At home, you can dip out the stock with a ladle or lift the whole pot and pour. Stop at the first sign of sediment, and drain the rest of your broth through several layers of cheesecloth to remove any impurities. If the end of the batch remains cloudy, reserve it for stews or casseroles, and use the clearer portion for soups and sauces. If you want to really go for broke, you can clarify your stock or broth to make consomme. Stir beaten egg whites into the chilled broth along with finely chopped mirepoix and an acidic ingredient such as tomatoes. The egg whites form a "raft" at the top that filters out any particles, leaving a crystalline broth.
Storing Stocks and Broths
The finished stock or broth should be cooled as quickly as you can. You can chill the whole pot by resting it in a sink full of ice water, or divide it into shallow containers and distribute them through your refrigerator and freezer. As with any other perishable food, you should use it or freeze it within the first few days. Package your stocks and broths in logical quantities, such as 1/2 cup, 1 cup or 2 cup measures. For small batches of sauce, freeze it in ice cube trays and use one or two cubes as needed.