Surprisingly a lot of people are not aware of puppy mills and what they are. Here is infomation for all of you that have asked what is a puppy mill.
Understand Puppy Mill Dogs

What is a Puppy Mill?
By the formal definition, a puppy mill is a large-scale breeding operation that produces large numbers of puppies for profit.
Puppy mills are inhumane in their treatment of breeding dogs and puppies.
Many are filthy, have no idea about or concern for the genetic implications of breeding or about what dogs require in terms of basic health care and socialization.
Dogs are kept in the most deplorable conditions and are confined to small wire cages for their entire lives. They are cramped and have as many as 10 dogs per cage. The floors are wired and cause injury to the dogs paws, most have splayed toes. The cages are stacked on top of one another from floor to celing. The dogs are coved in their own feces and urnine, are matted and have bad teeth.
Adopting A Puppy Mill Survior
Adopting a puppymill rescue is not for everyone. These dogs have not had a normal existence, and their contact with humans, if at all, could have been very unpleasant. In a nurturing environment, however, they have the potential to become the awesome companions they were meant to be. This does not happen overnight and can take many months. It requires patience and lots of love.
As a result of them living in their own waste they often do not understand the concept of housetraining. With a tremendous amount of consistency, patience, and love, this can be taught within a few months.
Puppymill rescues have the ability, if frightened, to buck and squeeze out of normal collars and bolt. If outside in an unsecured area, this may result in your dog getting loose collars need to be very tight, but make sure it is not so tight they can not breath. With toy breeds, a well-fitting harness can solve this problem.
Puppymill rescues are quite fearful of being picked up, this usually resulted in them being tossed or thrown from one cage to another. To feel secure, they need to be firmly held while gently rubbing their backs. Until they know and are secure with you, do not pet their heads or necks. Keep your hand on the dog while petting backwards (from the back toward the head) often can ease their fear and relax them.
It will take time for your puppymill rescue to feel secure with you. Do not be lulled into a false sense of security because your puppymill rescue no longer trembles when you pick him up. Just because you love him does not mean that he understands it. Accordingly, every precaution should be taken to ensure he is always in a secure environment
This information was taken from several sources