Hi welcome to Yorkietalk. Yorkshire terriers have only one class, they are part of the toy group in the AKC. Standard size yorkies are 7 pounds and under. We don't like to use the teacup word around here because many unethical breeders use that word to describe a very small 3lb and under yorkie. An ethical breeder will not use that word, so if you see someone selling teacup yorkies stay away from them.
I think it's very important to get to know the breeder and learn everything you can from them. The Yorkshire Club of American
http://www.ytca.org/breeder4.html lists breeders who are in good standing with the AKC. You are wise to do research ahead of time. Yorkies have been over bred in the past and have many heredity problems so it's best to go to a breeder that tests for this. It will save you money and heartache in the long run.