A purebred animal has a mother and a father of the same breed — both poodles, both Persian cats, both the same kind of cow. Breeders keep papers to show the family line stays inside that one breed. People care about this for dogs, cats, horses, and farm animals, sometimes because of looks or work, sometimes because a club requires it. Though actually, purebred is not a magic word for "better." Mixed animals can be healthy and kind; the word only talks about the family line. As a noun, a purebred is such an animal. Do not treat it as the same as Thoroughbred: that name with a capital T is also a famous horse breed, even though both words share the idea of a known line.