Generally speaking, if it would not have otherwise had a home, the home it was in was in some way dangerous (abusive, a hoarding situation, going to be put to sleep for no good reason, etc.), or if it comes from a place trying to save it from one of those two (a shelter or rescue), it's a rescue/adoption. If it's from a pet store, a breeder, or anybody else who is charging you any kind of money for it (adoption fees from legitimate shelters and rescues don't count), it's a purchase. If it's from a neighbor whose dog had puppies and needs to find homes for them, but they're not going to drown them in a river out back if you don't take it, it's just a puppy someone gave you. It may not be a purchase because you didn't give them money for it, but you didn't exactly rescue or adopt it either... you just... took it, I guess.
In the end, all dogs deserve good homes, so it's just a matter of semantics, really. While I would personally never get a dog from a pet store or backyard breeder (which includes anybody who lets their dog have puppies or gets their dog pregnant on purpose without being a legitimate breeder who genuinely knows what he/she is doing), the only type of dog-getting I have a real problem with is if you are the one letting your own dog have puppies. If you get one from a neighbor or a pet store or whatever, it's not what I would choose or recommend, but I'm not going to give you a hard time about it. If you get your dog pregnant so your children can experience the miracle of life or whatever, then... well... my mother told me that if I didn't have anything nice to say, I shouldn't say anything at all.