This is an excerpt from the YorkieTalk Library:
By nine to twelve weeks, puppy is refining reactions, developing social skills with littermates (appropriate interactions), and exploring the environment and objects. Puppy begins to focus on people; this is a good time to begin training.
Three to Six Months: Ranking Period
Puppy is most influenced by "playmates," which may now include those of other species.
Puppy begins to see and use ranking (dominance and submission) within the household (the puppy's "pack"), including humans.
Puppy begins teething (and associated chewing).
Here's a link to the whole post:
http://www.yorkietalk.com/forums/yorkietalk-yorkshire-terrier-library/176192-puppy-development-stages.html