"Who" refers exclusively to people. * Like: "The man WHO came to town." The only real attributive clause contention you'll see on this matter, is that between 'that' or 'which'. Like: "That is a book WHICH I need for class." vs. "That is a book THAT I need for the class." There's a (pretty weak) rule out there which says that THAT should be used with restrictive modifiers, and WHICH with nonrestrictive modifiers. Just ignore that rule, really; it's more often non-applicable than not. For smooth speech, just drop THAT or WHICH altogether, like: "That is a book I need for class." -- * For use of the relative pronoun as object, you can use THAT as well for humans: "The man WHOM I saw yesterday..." "The man THAT I saw yesterday..." Though 'whom,' in the above context, would be considered somewhat archaic these days. For best smooth speech, just drop WHOM/THAT here too. Like: "The man I saw yesterday..."