Who and whom are both interrogative pronouns (used to ask questions) and relative pronouns (used to refer to a noun or pronoun in the main clause).

Who is going? (interrogative)
Mr. Peabody is the one who is going. (relative, referring to one)

These pronouns may be either singular or plural in meaning.

Who is noding? (singular)
Whom do you prefer for these jobs? (plural)

Who (or whoever) is the nominative form. Use who whenever he, she, they, I, or we could be substituted in the “who” clause. (If in doubt, rearrange the clause as demonstrated below.)

Who is filling the nodeshell? (She is filling the nodeshell.)
Who shall I say is calling? (I shall say he is calling.)

Whom (or whomever) is the objective form. Use whom whenever him, her, them, me, or us could be substituted as the object of the verb or the object of a preposition in the “whom” clause.

Whom did you see today? (You did see her today.)
To whom were you /msg-ing? (You were /msg-ing him.)