What is a breed?
An actual 'breed' of dog doesn't just happen, it is carefully developed over at least seven generations to fix the desired characteristics. Especially when breeding purebred dogs, the puppies must be able to be identified as a member of the breed when compared to the standard and to other adults. In other words, each litter of puppies should resemble its parents, grandparents, great grandparents and so on.
It is important to know and realize that just by sight you can't necessarily prove, even with registration papers, that a dog is purebred. There are shady breeders, and worse even careless ones, that can and will sell crossbreeds of similar looking dogs as purebred when they aren't. Some examples this can happen with are say crosses between Lhasa Apsos and Shih Tzus, any of the small terriers, Siberian Huskies and Alaskan Malamutes, and quite a few others. Believe it or not, there are genetic tests now available to identify more than three dozen breeds, so really serious buyers have something to fall back on if suspicious...unfortunately it's not cheap, the run about $80 for a test.The advantage of owning a purebred dog is that the animal's appearance and character are pretty well set by generations of careful selection for particular traits. Puppy buyers can then more easily pick and choose the dog that will fit into their lifestyle and circumstances - active families can opt for energetic dogs; families with children can choose gentle dogs; people who live in apartments can select quieter dogs that need less exercise, etc.