Quote Originally Posted by Xuc Xac View Post
"Who" is a subject (it performs the action) and "whom" is an object (it "receives" the action of a verb or preposition).

Bill gave Jim a letter.

Who gave Jim a letter?

Bill gave whom a letter?

To whom did Bill give a letter?

Et cetera.
Note, however, that "Who" is used when the word undergoes movement (which, as an interrogative word, it almost always does). Someone who understands syntactic movement better than I do can probably explain why; Likely something to do with case-assignment processes.