a limited registration- the dog you are buying is AKC registered, but if it has pups, the pups can't be registered with the AKC. a limited registration can be lifted by the breeder at any time. a full registration- you can register the dogs litters
Limited Registration puppies are generally only intended to be pets (can also do obedience, agility, flyball... sports basically).
Full Registration puppies are usually show/breeding prospects. I personally won't use a breeder that gives full registration on each puppy because not every puppy is a show/breeding prospect and not every person should be a breeder. I think doing limited on some puppies and full on others show more disgression on the breeder's part and also is a sign that they care about their puppies futures.