Homemade stock has a flavour superior to and is far cheaper than anything that can be bought. Stock comes from the word stake and a good stock is a cooks support. It is the kitchens buttress carrying the elemental to the exquisite. This is the base for superb soups and demi-glaces to enrich gravies and sauces. Canned stock wont do it. Commercial consommes have a tenuous, boring flavour. Bouillon cubes yield little more than salt and colour. Powders are powerless. Good stock is worth making.
The flavour of stock depends upon ingredients and heat. Ours changes subtly every time we make it. Leftovers have the most potential: roasted chicken, lamb or beef bones tossed into a large pot with the remains of the evening salad (but no creamy salad dressings!), carrot tops, an extra roasted onion, a few cloves of garlic, mushroom stems, a few peppercorns, the vegetable cooking water, etc. Dont worry about grease or fat yet. Cover everything with cool water, put a lid on it, and heat this slowly until it just starts to boil. Skim any particles that have risen to the surface of the water. Put a lid on the pot and let it simmer. The longer the simmer the better the taste. A hard boil makes for a harsh, bony taste.
Check the pot regularly and add more water if necessary. The bones and vegetables should remain covered. After a few hours, take the stock off the heat. Spoon out the ingredients. Taste the liquid and add salt if you want. Let the stock cool to room temperature. Pour this into storage containers through a strainer or cheese cloth. Put the containers in the freezer. The fat will rise to the top and is easy to take off after it has chilled. The stock can be stored in the refrigerator for a week or in the freezer for several months. If you dont have much room (or want a more concentrated flavour), reduce the stock further reduced until there is just enough to fill an ice cube tray. It will be quite dark and thick. Store the frozen cubes in a baggie or container. One cube is the base for a rich gravy. Dilute it with about a cup of water for soup.