Im wondering though, whats the difference between a commonwealth and a state. In Fallout, they have 13 commonwealths. Whats the difference? Whats the advantage and disadvantges?

Some states today are still considered commonwealths such as Massachusetts and Virginia. "A nation or a state governed by the people; a republic"

"It was hoped that the new administrative division of the country would help create legislation broad enough to benefit those states with common regional concerns, but narrow enough not to affect those states with dissimilar interests or political cultures, which would help the nation better meet the challenges posed by the Communist threat to American democracy."

" In reality, it created even more political strife in the American government, as commonwealths typically did everything they could to promote their own interests at the expense of other commonwealths."

http://fallout.wikia.com/wiki/United_States_of_America#The_American_Commonwealths