The simple answer is that whom is the object form of who and should be used as an object. I can't explain it better than this link: http://theoatmeal.com/comics/who_vs_whom. The basic idea in the link is that you can try making the sentence with he/him and see which one works. "He" is the subject form and "him" is the object form. If "he" works then use "who", but if "him works then use "whom". Example: Who/whom brought you those flowers? Try this sentence with he and him. He brought you those flowers? This sounds good. Him brought you those flowers? This sounds bad. He is the subject form, so we will use a subject form pronoun. The correct sentence is "Who brought you those flowers?" Here's another example: Who/whom did you send the letter to? Try the sentence with he and him again. He did you send the letter to? Him did you send the letter to? This one is tough, and it might help if we try to write the sentence differently. Did you send the letter to he? Did you send the letter to him? The correct choice is the object form "him", so the original sentence should use the object form "whom" Whom did you send the letter to? The link I put explains it much better than I can.

WHOM is a pronoun used in place of "who" as a object of a verb or preposition. Ex-- Whom she has married.

Actually "whom" is very rarely used these days even in formal writing. a) Who did you give the book to? b) To whom did you give the book? c) To who did you give the book? .. (a) is what everybody would write or say. (b) is just about possible but sounds ridiculous. (c) is wrong.

The difference between who/whom is the same difference between he/him or she/her. who=subject form whom=object form "Who" performs actions, "whom" has actions performed on/to/for them.