limp d**k
Usually, whom is before a noun.
"I talk to whom I know."
And who, as above has posted, is used before a verb (and any possible adverbs in-between).
"Who are you?" Terrible idea. I can't think of an example where who is before a noun, but that is just the wrong logic entirely.
Okay, here we go. For everyone in this thread
GRAMMAR LESSON!"Who" is in the nominative case. That means this is for the "subject" of a clause.
"Whom" is in the accusative case. That means this is for the "object" of verbs and prepositions.
WARNING: THE COPULA IS NOT EXACTLY A VERB. The copula (aka the linking verb, the being verb, "to be" and all its crazy conjugated forms) DOES NOT TAKE AN ACCUSATIVE. Ever.
"You are who?"
NOT "You are whom?"