A puppymill is what supplies pet stores, pet wholesalers and brokers with puppies. The conditions in which the puppies and parents are kept in is usually sub-standard. You may find rows upon rows of small wire cages stacked on top of each other that the dogs live in. You will likely also see the care of these poor dogs to be less than acceptable. The reason for having these parents is to produce as many puppies as possible without regard to health. You will find the females pregnant all the time until they die or can no longer produce, then they will be discarded.
A puppymill is a terrible place for any dog. Don't buy from a pet store and support the puppymills. When you see that cute puppy in the window remember that his parents is on there next litter in that wire cage at the puppymill. I have heard of dogs missing legs, eyes infected, blindness, scares from fights, turning circles in there cages and other stories still being breed. This is no condition for any animal to be in.
Please use an ethical breeder when considering a puppy. I have heard so many horror stories that it is sickening. The link posted below is a single example of what goes on in a puppy mill. There are many more horrible stories that could be told. Please choose carefully when looking for a puppy.