The defination of a puppy mill is as follows: Establishment that breeds puppies for sale, typically many different breeds on an intensive basis and in conditions that regarded as inhumane.
A good breeder health tests first and foremost. They are careful to not breed closely related individual dogs. They will always ask the buyer questions about why they are interested in this breed usually via an application that asks many questions. A puppy mill, meanwhile, will simply sell you a puppy for the right price.