A Yorkshire Terrier and a Silky Terrier are two seperate breeds of dogs, although they look very similar.
http://www.akc.org/breeds/silky_terrier/index.cfm
Silkys are larger than yorkies, have larger ears, and a longer muzzle. A lot of yorkies look like silkys though because of pet-quality breeding. Also, there has been interbreeding of yorkies and silkys because a lot of people don't know the difference.
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~Magnifique Yorkies~
Purchasing from backyard breeders, pet shops, and puppymills perpetuates the suffering of other dogs.
Educate yourself and buy from reputable breeders or rescue.