www.millbusters.com is a great place to learn about puppy mills.
A puppy mill is a place where dogs are expected to continually reproduce without any regard for their health or the health of the puppies. Hundreds of dogs are kept in one location (there can be less). Their cages are stacked on top of each other and some don't have bottom so the dog's waste goes right through the floor of the cage and unto the floor to sweep up. So they sleep on metal bars. The whole building can be infested with bugs/flies. The dogs rarely, if ever, get medical care and their pups are sold to brokers. The brokers then sell to pet stores. That is why nobody should buy from pet stores. That is what it is supporting.