Feces is what is left over from the foods you eat. The food is chewed and soaked in acid, then enzymes are added, and the lining of the small intestine absorbs the nutrients. So feces is what is left of that process. Urine is the chemical waste of the body, much of which is water, uric acid and nitrogen-based compounds like ammonia. There might also be trace amounts of bilirubin and skin cells.