also titled, "Why fish don't have bums"

As metabolism breaks down macromolocules, nitrogenous waste is released. The form of this waste is highly toxic ammonia. In most aquatic animals, such as fish, this is the form in which it is released. Most of this is released in ionic form (NH4+) from the gills. Because the water quickly dilutes the ammonia, it is safe for fish to do.
On land, however, Ammonia is too concentrated to excrete in its natural form. ("Honey, some ammonia's dripping down your gill, better get it up quick") Mammals and many other land species produce urea from the liver instead, a substance 100 thousand times less toxic. In the liver, ammonia is combined with CO2 to make the urea, which is then transported to the liver (it's what's for dinner). This excretion of ammonia in changed form also helps land animals conserve water, a crucial adaption to living on land (and the reason for us having a bum).