Same as between he and him, she and her, I and me etc.
Who is she? She is Sally. Whom did you see? I saw her.
Where you would say she, use who; and where you would say her, use whom.
Another way of looking at it is to ask whether the the person is the subject of the sentence or object.
Who is that = subject
Whom did you see = object
That probably doesn't explain it clearly, but it's a start. Does this make any sense?