actually, there are not clear definition of puppy mill. But...
if a breeder in business for
profit, breeds
many breeds, has
lots of dogs, has litters
regularly, no matter if she sell to pet store/broker or not (although most of them do) many people describe it as puppy mill. Kennel condition can be from very good to horrible.
You can read more:
http://prisonersofgreed.org/kennel-examples.html