There is only one yorkshire terrier, it may vary in size anywhere from 1 1/2 pounds upto 20 pounds, but it is still a yorkishire terrier. The yorkshire terrier club of america sets the standard for the breed, just as any other breed club does and AKC uses the clubs standard on the ACK site. Yorkies were originally created through the breeding of different terriers. I have some of the information on my site about the breed history, you can see it here.
http://yorkieviews.com/yorkiehistory.htm