1. What or which person or persons: Who left?
2. Used as a relative pronoun to introduce a clause when the antecedent is a person or persons or one to whom personality is attributed: the visitor who came yesterday; our child, who is gifted; informed sources who denied the story.
3. The person or persons that; whoever: Who believes that will believe anything.

Usage Note: The traditional rules for choosing between who and whom are relatively simple but not always easy to apply. Who is used where a nominative pronoun such as I or he would be appropriate, that is, for the subject of a verb or for a predicate nominative; whom is used for a direct or indirect object or for the object of a preposition. Thus, we write the actor who played Hamlet was there, since who is the subject of played; and Whom do you like best? because whom is the object of the verb like and To whom did you give the letter? because whom is the object of the preposition to. · It is more difficult, however, to apply these rules in complicated sentences, particularly when who or whom is separated from the verb or preposition that determines its form. Intervening words may make it difficult to see that Who do you think is the best candidate? requires who as the subject of the verb is (not whom as the object of think) and The man whom the papers criticized did not show up requires whom as the object of the verb criticized (not who as the subject of showed up). Highly complex sentences such as I met the man whom the government had tried to get France to extradite require careful analysis—in this case, to determine that whom should be chosen as the object of the verb extradite, several clauses away. It is thus not surprising that writers from Shakespeare onward have often interchanged who and whom. Nevertheless, the distinction remains a hallmark of formal style. · In speech and informal writing, however, considerations other than strict grammatical correctness often come into play. Who may sound more natural than whom in a sentence such as Who did John say he was going to support? —though it is incorrect according to the traditional rules. In general, who tends to predominate over whom in informal contexts. Whom may sound stuffy even when correctly used, and when used where who would be correct, as in Whom shall I say is calling? whom may betray grammatical ignorance. · Similarly, though traditionalists will insist on whom when the relative pronoun is the object of a preposition that ends a sentence, grammarians since Noah Webster have argued that the excessive formality of whom is at odds with the relative informality associated with this construction; thus they contend that a sentence such as Who did you give it to? should be regarded as entirely acceptable. · Some grammarians have argued that only who and not that should be used to introduce a restrictive relative clause that identifies a person. This restriction has no basis either in logic or in the usage of the best writers; it is entirely acceptable to write either the woman that wanted to talk to you or the woman who wanted to talk to you. · The grammatical rules governing the use of who and whom in formal writing apply equally to whoever and whomever and are similarly often ignored in speech and informal writing. See Usage Notes at else, that, whose.