Two completely different breeds.
Silky Terriers have Yorkie in them so they look very similar. Silkies are larger (which is why a lot of people who own oversized Yorkies get questions about their "Silkies")
www.akc.org
go to breeds
go to toy
find info about both
Confusion may also come by the term we use to describe the fur on our babies. The desired coat is "silky" It is easy to care for, doesn't tangle and gives the Yorkie that beautiful show girl look.
Many Yorkies have cotton coats. They require more care to keep them straight, shiny and tangle free.