The third link is probably the best to give you an explanation or definition of a puppymill.
I seemed to have moved to a place where puppymills are the norm (Missouri)...very different from California where we mostly just had to deal with BYB (backyard breeders - people that breed their pet to make some money or because 'everyone wants a dog like....'

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An easy definition of a puppymill is an operation that produces puppies to sell for commercial purposes. These operations commonly have many different breeds of dogs available. The conditions these dogs are kept in is usually not the best - or even close to acceptable. They are kept in small cages, sometimes stacked on each other. Also, the other thing that I REALLY noticed when I went to a place that ended up being a puppymill (went with a friend that wanted a puppy and went only so I could talk her out of it....thankfully that worked), was that they didn't allow people back where the dogs were kept, they brought the puppies and parents to us.