What Are They?
Puppy mills are places where dogs are bred repetitively, so the breeders can make profit off of the puppies. There are so many dogs that living conditions become dirty and grim. Nearly all dogs will be diseased in some way.
Plus, these dogs cost thousands of dollars and many owners often end up spending thousands more paying for their medical bills.
Here's a link to more information from the Humane Society of the United States.
How Can They Be Stopped?
Puppy mills can easily be stopped if everyone is aware of them. Here are a few simple ways you can help stop puppy mills.
Educate: If someone you know is getting a puppy, make sure they are getting one from a shelter or a responsible breeder. This is the number one cause of puppy mills, because if people buy the puppies, there are reasons to breed them!
Donate and Participate: If you're the type of person who likes to donate, consider donating to a non-profit organization that takes in puppy mill rescues. These organizations rely solely on donations, and puppy mill dogs are expensive to rescue. If you'd rather participate, try volunteering for a shelter or a rescue group. Or just come to races like the Puppy Power 5k.
Inform Yourself and Be Creative: The more you learn about puppy mills, the more ideas you'll get about how to stop them. Two good books for this are: Ellen Mile's "Chewy and Chica" (geared towards kids) and Carol Bradley's "Saving Gracie" (more intense, but very informative). And then get creative!
Here's a link to the Humane Society of the United State's list of ideas.
How can you tell if a breeder is responsible? Here are some ways:
They will let you see where the dog lives and see the dogs parents. Any breeder who just wants to meet you at a parking lot or a coffee shop is almost always not responsible.
They will let you see the puppy and visit with him/her before you buy them. They should also do some sort of a background check to make sure you're a good fit for the dog.
They will tell you of any preexisting health conditions. While puppy mill puppies look adorable and somewhat healthy on the outside, they often have internal diseases and problems. A responsible breeder will make sure you are fully aware of these complications before you fork over the money.
Here's a link to the Humane Society of the United State's page about responsible breeders.