Who is the nominative form. Whom is the objective form. Who is the subject of a sentence (Who is calling?) or the predicative nominative of the verb to be (The leader is who? Who is the leader?) Whom is the object of a sentence (the thing something is done to) or of a prepositional phrase (The rock hit whom? The book is about whom? To whom was the award given?) The most common error, even made by journalists, is to use whomever as the subject of an objective clause. ("Give it to whomever wants it" is WRONG. It should be "Give it to whoever wants it" just as it would be "Give it to who wants it." In this case, who and whoever are in the nominative case because they are the subjects of the clause who/whoever wants it. "Give it to whom you like", however, is correct, because whom is the object of the clause "you like whom" with the word order reversed) In the U.S., people use who 99% of the time when whom would be traditionally correct. To use whom incorrectly (even correctly sometimes) sounds pretentious--like you're trying to show off your grammar skills. If in doubt, use who. In the U.S at least, almost no one will notice, and no one will criticize you.