Puppy mills are basically breeders who are in it for the money and not the betterment of the breed. A lot of times they'll have an obsene number of dogs in deplorable conditions, often several different breeds. They don't check health or temperament before breeding. If you go to puppy mill rescue sites, the pathetic shape of the overbred unhealthy dogs will make you cry. Some sites have pictures of puppy mills that are in such disgusting condition you would not believe it. Pet stores that sell puppies get them from puppy mills.