Who | Whom
Who has two forms; a subject form (who) and an object form (whom). A subject is the person who does the action, whilst the object is the recipient of it. In the sentence
"I threw the ball at him"
'I' is the subject, and 'him' is the object. If you wished to use who and whom in this sentence, it would end up looking like this:
"Who threw the ball at whom?"
You might find this a bit proper, and indeed, most people would probably say "who threw the ball at who?". This is also quite acceptable. You might find this surprising, but the simple fact is that the use of whom is dying out, and is increasingly becoming an archaic form. More and more educated writers are now using who for both subject and object.
That's not to say that you cannot use it or should disregard it, but be very careful that you do not to make the mistake of using whom as a subject. A quick way to determine whether you should use who or whom is to think whether you could replace whom in your sentence with him/her. In the examples given above above, this yields
"Who threw the ball at him?"
Which gives a grammatically correct result. If you really aren't sure which form to use, then stay with who, which is quite acceptable in both cases for most people.
FOOTNOTE: You may wonder if whose is relevant to this discussion. In some ways it is - whilst who and whom represent the subject and object forms respectively, whose represents the possessive case. However, the main problem people have with whose is confusing it with who's, and so this has been dealt with on another page.