Whom is the objective form of who. It is used when you are referencing the object of a verb rather than the subject of it, such as "Whom did you talk to?" In this sentence "You" is the subject, the verb being talk, and as it is the object of the verb that is in question, whom is used rather than who. One important note is that, atleast in the United States, this word is almost completely obsolete. Most native English speakers don't know the difference, and they will rarely if ever use the word in favor of simply using who for both the object and subject. Personally, I only use it in academic writing, never in speech.

pron Definition: 1. introduces question: used to introduce a question asking about the name or identity of a person or people Who's that at the door? Who did you see there? 2. introduces relative clause: used to introduce a relative clause giving information about a person or people meals for people who are too busy to cook [ Old English hwā< Indo-European, "who, what"] Word Usage who or whom? Whom has fallen into disuse in everyday speech, with who taking its place, especially in British English. Do you remember whom you saw? is more usually expressed as Do you remember who you saw?, and whom is omitted when it is associated with a preposition: the man I was talking to rather than the man to whom I was talking. However, in formal contexts, whom is still preferred by careful writers. Note that whom is incorrect in sentences such as The woman who we thought was dead is still alive: who is the subject of was, not the object of thought (We thought that she was dead...).