Who vs Whom

One does the action; one receives it.

• WHO — the subject (does the action). Who ate the cake?
• WHOM — the object (receives the action). To whom did you give it?

Quick tip: if you could answer with “he/she,” use who; if “him/her,” use whom (both end in m). In everyday speech most people just say “who” — “whom” is the formal one.