by-products are any parts left over from animals that aren't fit for human consumption. This can include feathers, bones, feet, blood, and sawdust from the floor.

Fillers are ingredients added to a food that have little to NO nutritional value but serve as a cheap ingredient to add and make your pets "feel" like they are full. Fillers usually are not digested and come out as nice little packages on your front lawn. The MOST commonly used filler is any form of corn!

Here are the ingredients for IAMS Lamb Meal & Rice formula. "bad" or "questional" ingredients (from my view point) I have bolded:

Lamb Meal, Brewers Rice, Corn Meal, Ground Whole Grain Sorghum, Ground Whole Grain Barley, Chicken Fat (preserved with mixed Tocopherols, a source of vitamin E, and Citric Acid), Fish Meal (source of fish oil), Chicken By-Product Meal, Corn Grits, Dried Beet Pulp (sugar removed), Natural Chicken Flavor, Potassium Chloride, Dried Egg Product, Brewers Dried Yeast, Salt, Monosodium Phosphate, Sodium Hexametaphosphate, Vitamins (Vitamin E Supplement, Ascorbic Acid, Vitamin A Acetate, Calcium Pantothenate, Biotin, Thiamine Mononitrate (source of vitamin B1), Vitamin B12 Supplement, Niacin, Riboflavin Supplement (source of vitamin B2), Inositol, Pyridoxine Hydrochloride (source of vitamin B6), Vitamin D3 Supplement, Folic Acid), Choline Chloride, Minerals (Ferrous Sulfate, Zinc Oxide, Manganese Sulfate, Copper Sulfate, Manganous Oxide, Potassium Iodide, Cobalt Carbonate), DL-Methionine, Rosemary Extract
When reading an ingredient list, ingredients are listed from the highest weight to the lowest. So the first ingredient makes up the majority of the food whereas the last ingredient makes up the very smallest percentage. Make sense??

If you want more information on reading lables, google the term or here are a few sites that I have found helpful and have helped teach me!
http://www.asuperiorgsd.com/feeding.html
http://www.asuperiorgsd.com/dog.food.html
http://home.att.net/~wdcusick/011.html