IMO, a puppymill (I live down the road from one that breeds greyhounds
) is anyone who mass produces dogs for profit. The one I live near keeps their kennels very clean, but to see those puppies sitting there all alone, undersocialized, and to eventually be turned into a race dog (and later to be thrown away because he won't run "fast enough") makes me sick. They breed tons and tons of dogs, pumping them out ASAP, just to make money off them.
BYB's are a little different. I knew one personally, but didn't know enough about the subject to realize just what she was doing. She was a wonderful person, but she was breeding irresponsibly. Her stud dog, Fester, had a bad leg. Everyone was warned not to pick him up, because his leg was messed up. Now why would you breed a pug with a bad leg????? Her little female, Zoe, was seemingly healthy. They had great temperments, and I enjoyed going to her house and playing with them, but they were not breeding quality. She didn't show them, she didn't health test them (why bother? She already knew the male was unhealthy), and, as far as I know, her dogs were never seperated, so her female was bred at every heat. Her dogs were treated like pets, and she did make an effort to find good homes for her $800 dollar pups
A good breeder IMO is someone who breeds for nothing but love for the breed, and to better it. I personally like breeders who compete with their dogs, to be sure the dog is a good example of it's breed. Their puppies are well socialized, their dogs well loved. The parents are always health tested, so as to avoid breeding puppies with inherited conditions such as Hip Dysplasia, heart defects, etc.
These are just my own opinions, BTW.