A mill is a breeders dream situation. It's basically putting animals together until the female is pregnant. Once she has the babies, they are kept with the mother a minimum amount of time until they can be force weaned, and sold off to pet stores or private customers or put into their own cages and start the breeding process all over again. No love or caring, tiny breeding cages, short life spans.
Cavy spirit has a good article on a mill called Hollister. A lot of GL members seem to have pigs from Hollister, they where rescued, but many have had life long health problems, and many have passed away.