It literally means a place that churns out puppies as a product.
The emphasis is on making profit. To do that, the dogs have as many puppies as they can, then when they are worn out they are either dumped in rescue, given away or put down.
The dogs are typically kept in battery cages like battery hens. They don't get let out or taken for walks. Or groomed, or seen by a vet.
The adult dogs may have any genetic disorder, they will still be bred from. Imported foreign bred puppies are a Rabies risk. Puppies are sold with diseases such as Parvo virus, many die.
The dogs may still be KC or AKC registered and full pedigree.
Typically the pups are either sold through a pet shop, or by local people through local free ad papers. They usually give a sob story that the mother died and the pups were hand reared.
Or they breed their own dog and buy in extra pups to sell as ringers.
They also get sold at car boot sales, auctions, in pubs and at motorway service stations.
Stop puppy farming
http://www.puppylovecampaigns.org/
http://www.rspca.org.uk/getinvolved/campaigns/companion/dogwelfare/borntosuffer/pedigree
http://www.youtube.com/results?search_query=puppy+mills+exposed&sm=1
http://www.pets4homes.co.uk/pet-advice/puppy-farms-how-and-why-to-avoid-buying-a-farmed-puppy.html
http://cariadcampaign.wordpress.com/
Eastern European puppy farms supply UK pet shops;
http://www.mirror.co.uk/news/uk-news/pups-trafficked-young-dogs-are-being-1391112
Would the money be better spent in supplying humane euthanasia to a larger number of animals?
http://www.secondchancerescuekennels.co.uk/
http://bepuppyfarmaware.webs.com/yourstories.htm