There is some contention over the definition of the term, "puppymill". A common definition describes a puppymill as a "substandard breeding facility that fails to adhere to the Animal Welfare Act, and which tries to avoid state and/or federal inspections". My definition is broader:
Puppymill: A mass breeding facility, the sole purpose of which is monetary profit, with little or no regard to the health and well-being of the animals therein.
This being said, there are varying degrees of puppymills. I know of large puppymills with heated, air-conditioned indoor/outdoor runs for the dogs - much superior to the opposite end of the spectrum, which generally consists of rabbit hutches in barns with little light or ventilation. Yet, these "better" puppymills still breed females each heat cycle, and offer little or no veterinary care for the animals.
The IL-MO Rescue web site, has some very useful information on puppymills. We wish that anyone considering either adopting or working with puppymill dogs would first read "Understanding Behavior of The Puppymill Survivor". Although few puppymill survivors have all the behaviors described, all have some of them.
All the information about puppymills you could possibly want can be found on the following sites:
No Puppymills.com: Hands down, the very best informative site on the worldwide web.
Heart United For Animals: This shelter in Nebraska is not only Nirvana for homeless pets, they are leaders in the fight against puppymills. We support them 100%, and rely on them for anti-puppymill strategy.
Critter Haven, Inc.: The Caring Place: Good Info on many topic, including mills.
