"Who" is a subject pronoun that either asks the question of "Which person?" or clarifies which person.
Who did that?
She is the one who was dating Ed.
"Whom" is "the objective case of "Who" and is properly used where the word functions as an object" (p. 931 The Pocket Oxford American Dictionary of Current English. New York, NY: Oxford University Press; 2002). In the sentences below, you can see that "whom" is the direct object of each sentence. "Whom" is receiving the action of the verb.
To whom did you send the gift card?
My sister is the one of whom you are speaking.