Abarim Publications' online Biblical Hebrew Dictionary
The two interrogative pronouns מה (ma) and מי (mi) occur frequently in the Biblical narrative; מה (ma) slightly more often than מי (mi). The difference between the two is that the pronoun מה (ma) asks for a quality (what, as in "what is your name?"), whereas מי (mi) asks for identity (who, as in "who are you?"). Here and there it occurs that where English would use 'what', Hebrew uses 'who', for instance when asking about the identity of a nation (Deuteronomy 4:8; 'who of the nations...'), or simply that of whoever ('which one' or 'who of the...').
Note that our word מי (mi) is spelled the same as the construct-plural form of the word מים (mayim), meaning waters. Hence מי also means 'waters of'.