The short explanation is that the Jack Russell Terrier was never actually considered a breed until the last decade. There have been many different names used to differentiate between the taller version usually ranging from (12-15") and the shorter version (typically 9-10"-12"). The shorter Jack Russell Terriers are known by many names like: Shorty Jack Russells, Short-legged Jack Russells, Shorties, English Jack Russell Terriers, Russell Terriers, Irish Jack Russell Terriers, Miniature Jack Russell and of course, simply Jack Russell Terrier puppies. While there are some differences in the Jack Russell Terrier world, what matters most, is not what type/size/style of Jack Russell you are considering - But rather the health and genetics behind the line and the breeding and socialization program the breeder practices.