You can learn all about Mills by searching the net using key words...PUPPY MILL....
There is a wealth of information out there and most all of it would make you sick to read...be warned...they can be VERY graphic and make you want to cry.
Here's some to start you off....
http://www.stoppuppymills.org/ http://www.prisonersofgreed.org/
ALSO...Please know MOST Pet Stores buy all their puppies from these Mills. This is why you see so many posts saying DO NOT BUY FROM PET STORES..
good luck reading ...it's a huge eye opener