Pronoun

1. What person or people; which person or people, as the object of a verb.

   Whom did you ask?

2. What person or people; which person or people, as the object of a preposition.

   To whom are you referring?

   With whom were you talking?

3. Him; her; them (used as a relative pronoun to refer to a previously mentioned person or people.)

   He's a person with whom I work.

   We have ten employees, of whom half are carpenters.

Usage notes

In both spoken and most written language, who is often used in place of whom, even as an object, although some prescriptivists regard such usage as incorrect. In US English, the use of whom is characteristic of a formal style and—to some—sounds stilted in informal conversation. Whom is more common in UK English, particularly after a preposition (with whom, from whom etc).