"You are a woman."
Translation:Tu sei una donna.
So, what I've understood so far is that "voi" is only used for "you" for two or more people, informally. The formal pronoun for "you" is actually "Lei" (capitalized) for a one male or female. Read more here: http://italian.about.com/cs/grammar/ht/formalinformal.htm
Because that would be plural.
Tu sei una donna = You are a woman (look, she is a woman!) Voi siete donne = You are women (look, they are women!)
Voi siete una donna = You are women (look, they are a woman!).
Now I'm imagining a group of performers pretending to be one woman in some kind of a charade game...