Many English speakers do not know the difference between who and whom. In some places, it hardly matters, because using who when you should use whom is so common that it’s not even considered much of a mistake. But for those who want to know the difference between who and whom, here is an explanation.
Who is an interrogative pronoun and is used in place of the subject of a question.
Who can also be used in statements, in place of the subject of a clause.
Whom is also an interrogative pronoun, but it is used in place of the object of a question.
And whom can be used in statements, in place of the object of a clause.
Whom is always the correct choice after a preposition.
The Bottom Line
The difference between who and whom is exactly the same as the difference between I and me, he and him, she and her, etc. Who, like I, he, and she, is a subject – it is the person performing the action of the verb. Whom, like me, him, and her, is an object – it is the person to/about/for whom the action is being done. Whom is also the correct choice after a preposition: with whom, one of whom, not "with who, one of who."
Sometimes it helps to rewrite the sentence and/or replace who/whom with another pronoun so that you can see the relationships more clearly.