A puppy mill is a breeding establishment in which the breeder sells puppies for profit.
People who have no knowledge of the dogs they are breeding usually run puppy mills and they breed puppies in mass quantities. Not all puppy mills are this way, but a lot are. Some respectable breeders have been labeled as puppy mills even when the conditions are humane and healthy.
Puppy mills began popping up after World War II. Farmers who needed to find new ways to make money were encouraged to breed puppies. A lot of them had no knowledge of the animals they were breeding and were usually low on cash so they began breeding the dogs in old buildings that already existed on their land. As the demand for puppies grew, so did the rate at which these farmers bred the dogs. With all the dogs and little space to house them, these mill operators began allowing their place to become overcrowded.
After the passage of animal rights laws, some of the mills cleaned up their act and are considered to be somewhat respectable breeders. A lot of the mills though still run in the same foul conditions that are not only cruel to the animals but are very unhealthy. Animals are forced to live in cramped conditions and are required to have litter after litter. Most of the breeder bitches are still nursing a litter while giving birth to a new one. There is little recovery time for these dogs so their life span is cut short.
Because of all the crowding and lack of care, most of the dogs from a puppy mill are unhealthy and shouldn't be breeding. Because of this the puppies are often born with health problems and are sold via pet stores to unsuspecting families. People who purchase the unhealthy puppy usually wind up with a high vet bill or worse yet, the heartbreak of losing the puppy.
There are laws to protect dogs against the cruelty of puppy mills, but because it is so profitable for the breeders they keep popping up. The best way to stop this is to cut it off at the source. Avoid buying puppies from pet stores. When buying a puppy, make sure it's from a licensed breeder. Making puppy mills unprofitable will eventually put an end to the cruelty of unlicensed overcrowded puppy mills.