"Buff" typically refers to the color of a dog's coat. Buff dogs are usually a light brown or tan color but can also be golden, blond, tan or red. Buff is also often used to describe the coat color of cats.
Besides being used as a description of coat color, "buff" may also be used to refer to a dog's muscular physique, just as it is often used as a slang word to refer to a human's physique. For example, BuffDogs.com belongs to a dog breeder in Denver, Colo., who is known for breeding muscular bulldogs, and BuffDog Canine Fitness Center in Santa Ana, Calif., specializes in canine fitness and agility.