Cat food has something in it that is essential to a cat's digestive system. Dog food lacks that ingredient. I don't know if that's true or not, but I've always heard it. 
Too tired to look it up right now.
It's more than one thing Shasta - cats need a diet quite different from a dog - and it's important for their overall survival, not just their digestive system. Cats have to eat meat (true carnivores), dogs do eat meat, but they can actually live without it. Dogs can make certain amino acids that cats can't. Cats need to get them from their diet.
It's okay for cats to eat some dog food, as long as it's not the bulk of their diet, just like we can't eat only french fries.
'kay. public service annoucement from resident veterinarian is over with.
