what is 'dog food'?

When Penny was recovering from her cruciate operation, I downloaded the incredibly helpful and FREE booklet from Dr James St Clair about post-operative care. (I'm not sure it's still free, but I hope it is.)
I have the greatest respect for him as a vet, even though I've never met him. (I live on the other side of the planet from him.) So when I read a post by him about the pet food industry, I expected  it to be good.

And it is.